The Two-State Solution: Old Standby, Perhaps, but Imperative
There is a reason that the two-state solution has, in one form or another, stayed around … and around …. and around, since its first iteration in the British Peel Plan, also known as the first partition plan, in 1937. It is inconceivable that anything else can get us out of the current logjam. Not that, as we know, the two-state solution is a panacea. As Churchill's cliché about democracy had it; it's the worst system, except for the alternatives.
There is a difference between Churchill's formulation and the two-state solution. In fact there are other solutions that might work: Federation, confederation, bi-national state, cantonization, etc., which are all variations on the old one-state solution. All of them have their passionate and often well-meaning advocates and carefully thought-out programs that deal with the real issues, and many are based on models that have worked or do work elsewhere. Their common problem: You can't get there from here.
Aesop's fable of the cat and mice explains the problem clearly, and there's no need to label who is who. The mice in a house were complaining that the cat was catching and killing them, and they were facing annihilation. They needed a new approach. Finally, one suggested that they put a bell around the cat's neck, so the sound would warn them whenever it came around. The proposal was examined carefully, all thought it a superb suggestion, and they were ready for implementation. Finally, the oldest, most grizzled mouse asked cynically, "Who will bell the cat?"
In other words, how do we create a one-state solution, which is opposed by virtually all Israelis, when we cannot get to a two-state solution, with far broader, if partially nominal, support?
Most plans I have seen content themselves with laying out the advantages of and explaining how, when it came into force, it would ameliorate the situation immediately, perhaps even for both sides. But often there is not even an attempt to lay out a realistic process for adoption and implementation, since its advantages are supposed to be self-evident. In other words, the UN or the U.S. or the "international community" is expected to impose it.

The Real World

That isn't how the real world works. That world bears no resemblance to the world that has existed since the 1930's, certainly since Israel was founded in 1948. Israel was founded on a passionate belief in the absolute necessity of a Jewish state. The issue is no longer, for those who want a solution to the conflict, the rightness or wrongness of that solution, or its clear injustice to the Palestinian people, collectively and individually. Israel will not allow its nationhood to be fundamentally compromised, by the U.S. or anyone else. The U.S., under any imaginable leadership, will not force Israel to abandon the idea of a Jewish state. And until the UN becomes a world government with the coercive power to fight wars - something hard to imagine for a long time to come, if ever - it cannot enforce such a policy, even if it were enacted though some sort of "Uniting for Peace" - type resolution, which enabled the UN to fight the Korean War in 1950-53.
While in my view and in that of many others, it is politically possible and feasible, not to mention imperative, for the U.S. to play a far stronger role in the peace process, including encouragement of both sides to take steps they would prefer not to and probably would not agree to in bilateral negotiations, the suggestions above are of a very different order of magnitude. If you posit a U.S., or some other world power, doing this, you might as easily posit a supernatural force, or a change of heart towards goodness on the part of all men and women, or the Messiah coming. All of these are well within the realm of imagination but not within the purview of serious political analysis.

Improbable, but it happened

Of course, determining what is "politically possible" is not an exact science. And it is not unusual for pundits to be flat wrong, What seemed unattainable and fantastic can, a generation later, turn out to be - surprise! - within the realm of political possibility.
The best single example I know is one that every reader of the Palestine-Israel Journal is familiar with, i.e., the Zionist movement's successful campaign to establish the State of Israel. In 1897, Theodore Herzl declared, "In Basle I established the Jewish State. If I were to say it publicly today, the response would be laughter from all directions. Perhaps in another five years, 50 years at the most, everyone will recognize it".
"Laughter" was a euphemism. Any sober political analyst in 1897 would have said, in my view, that the chance of the dispersed and persecuted Jewish people establishing a state in 50 years was insane, on the par with something demonstrably impossible, like reaching the moon. Yet Israel was established 51 years later (and the moon was reached only 21 years after that, but that's a different issue). Is a one-state (or any non-two-state) solution any more unlikely than that?
In my view, it is, with a couple of caveats. Between 1897 and 1948, the world went through two world wars and the Nazi Holocaust, and was well on the way to dismantling the Euro-centric system of colonialism and imperialism (while many Arabs may see the establishment of Israel as part of that system, at the time most Europeans and Americans emphatically did not). The Zionist movement, with a combination of extraordinary luck, shrewd timing, and pragmatism was positioned to use these events and catastrophes to achieve its aim of a Jewish state. It was improbable, but it happened.

Is a revolution possible?

Could not a similar cascade of events theoretically bring about a one-state or (non-two-state) solution? Of course. But only if you posit a world order fundamentally different from today's, with the neutralization of the United States (or a complete change of its political culture and perception of its interests), the non-existence of an Israeli nuclear capacity, and a world polity (or new power) both willing and able to force a one-state solution on a recalcitrant Israel. Then it is possible. All of these are conceivable, and it is possible that some equally unlikely scenarios will actually come to pass. But the chance of a revolution in the world order both occurring and being able to bring about such a specific change in a usable form is highly unlikely. Revolutions are blunt and clumsy instruments, and the ultimate failure of most of them in the 20th century testifies to their weaknesses.
Focusing on the apocalyptic has its costs. It is hard to pursue evolutionary, consensual change while simultaneously seeking revolutionary, coercive transformation. It tends to destroy your credibility. And the political momentum necessary for one is usually very different from that required for the other.
In 2003-04 there was a renewed spate of interest on the part of some members of the Jewish left in the one-state solution. Veteran historians and analysts such as Prof. Tony Judt and Dr. Meron Benvenisti argued that the two-state solution was simply no longer attainable, and that the only alternative was some form of one-state. Benvenisti, of course, has been making similar arguments since the 1980's. With genuine respect for both of them, I don't think their analysis holds water.

Two-state solution or no solution

The alternative to the two-state solution is not one-state but rather no solution, which is what we have had for the last sixty years. Or rather, a partial solution, benefiting Israel and doing little for the Palestinians. The inertia of stalemate is an extremely powerful force. Given the unlikelihood of war accomplishing the aim of restructuring Israel and Palestine into some form of one state, or of Israel imploding like the Soviet Empire (a highly implausible scenario, despite some recent statements of pessimism from Prime Minister Olmert and other Israelis), it is more likely that, in the absence of two-states, twenty years from now we will still have some version of what we see today.
The seeming post-Oslo decline of the two-state impulse does not really strengthen the move towards one-state, though it does increase Palestinian desperation. The growth of settlements, refusal to share Jerusalem (though this has simultaneously become more popular in some unlikely sectors, such as with the current Prime and Foreign Ministers) do not make a one-state solution any more plausible. Who will bell the cat?

Two-states - a door to other options

Does that mean that confederation, bi-nationalism, or some other solution that recognizes the need for political and economic cooperation and equality between Israelis and Palestinians is impossible? Not at all. But the only possible road to them is through the two-state solution. It is unlikely that there is any other way.
The (viable) two-state solution is the clearest possible statement of a pragmatic willingness to accept the permanent national presence of the "other" in the Middle East. Declarations of Principles and revisions of charters don't cut it. In today's world, as post-nationalist as (Western) Europe, at least, may be, self-determination is the gold standard of acceptance and responsibility. "Post" means "after". Europe could not reach post-nationalism without first attaining national responsibility, though some of the states now surrendering elements of their sovereignty only received it, theoretically or practically, in 1989-90, others a bit earlier in 1919.
Once the two-state solution works (I'm convinced it will, but others are understandably skeptical), then other possibilities emerge. Once real circumstances show Israelis and Palestinians - and their neighbors and partisans - that the other side is genuinely willing to focus on things other than eliminating them, other options will open up. In other words, the two-state solution is an open, not a closed door. But further developments must await the construction of trust between the two sides, on a societal as well as a governmental level, which is now at an abysmally low point.
I'm of the majority school that is deeply pessimistic at the moment. I see scenarios for positive movement - a number of them - but I am less than convinced that any of them will ripen soon, though the real possibilities exist. However, I see no plausible scenarios for a consensual one-state solution - whatever the variation. The one-state solution is not a default option. It will not happen just because two-state is stalemated. Getting there would require surmounting all the two-state hurdles - and then some. It is not a serious option.