I like just about half of Nate Silver's pushback against the idea that a third party serious run for the presidency is improbable (roundup here from Brendan Nyhan; Silver doesn't take on the question of whether or not such an idea would be a good thing, just whether it's plausible).
I basically agree with Silver that it's not at all improbable that a "serious" third-party candidate could emerge in 2012. I do think he overstates the chances that candidate would have of winning, however. As Silver notes, serious third party campaigns are by no means unusual. What I'd say is that the proper way to think about them is by dividing the conditions for such a candidacy into two parts, supply and demand. Demand for a third party is easy to explain; while Silver has quite a few conditions, we could boil all of it down rapidly to one: demand for a third party candidacy is caused by unpopular presidents. That links each of the postwar cases (1948, 1968, 1980, 1992) but one, the Perot echo candidacy in 1996. If the president has a high approval rating, you're not going to get a strong third party run. But it's also true that demand is not sufficient; you also need someone willing to do it who has sufficient credentials that she will get Treated Seriously by the press, at least for a while. Perot got that treatment because he was rich; John Anderson in 1980 was Taken Seriously after he attracted a lot of attention in the GOP primaries. Most likely, any sitting Governor or Senator would qualify.
So, if Barack Obama's approval ratings take a turn for the worse over the next year, I do think the conditions will be there for a third-party run, if someone decides to go for it. I disagree that such a candidate would have a reasonable chance of winning; I don't think Ross Perot in 1992 ever had a reasonable chance of winning. The trick here is to think about voters and party identification. It is true that many voters do not think of themselves, and do not say that they are, party voters. As John Sides never hesitates to remind us, however, most of those people actually do act like partisans. Here's what basically happens. If a president, such as Jimmy Carter 1980 or George H.W. Bush 1992, is unpopular, that means that there's going to be a large group of people who think of themselves as voting for the person, not the party, and who also don't want to vote for the incumbent. What do they think of the out-party challenger? Well, they're not explicit partisans, so they don't simply rally to Ronald Reagan or Bill Clinton. Indeed, there's probably an ongoing or just completed tough fight for the nomination on that side, so they've heard lots of negative things about the surviving candidate...last they heard, even a lot of Democrats don't seem to like Clinton (or Republicans, Reagan). So you'll get a fair number who will say they will vote for a third party candidate, who at that point probably has the benefit of having avoided any negative publicity at all. However, by the time the conventions come, the out-party suddenly seems, despite all (media) predictions to the contrary, to be unanimously enthusiastic about their candidate, who of course gets a multiday infomercial that even now is still broadcast pretty widely. Moreover, the "independent" Republican is going to be watching Fox News (now shifting to a 2012 scenario) and hearing a lot of really good things about the GOP candidate, and some disturbing information about the third-party candidate, who suddenly looks a lot less new and shiny, and more like politics-as-usual. Over on the other side, MSNBC hosts who may spend 2011 beating on Obama for deviations from the party line, leading some "independent" Dems to flirt with a third party candidate, will spend fall 2012 talking about how scary the GOP nominee is, and how Nader caused Iraq, and those independent Dems are going to float back to the Democratic president. In our hypothetical, Obama is unpopular, so that requires another group -- people who normally vote Democratic but have sworn off Obama. For them, it's probably easier to use the third-party candidate as a placeholder for a while, on their way to the actual taboo of voting for a Republican, just as Reagan Democrats did in 1980.
And that's without getting into a lot of the technical stuff, such as ballot access laws, or the advantages the established parties have in ready-to-go get-out-the-vote campaigns. Essentially, there's just a lot of machinery in the political system designed to push people to the major parties at the end of the campaign, and that's what we should expect will happen at the end of the road, whatever things look like in March or July.