At least, that's what the people at Intrade think at the moment. Acquisition by New Years Eve '12-13, 1.5%, and by NYE '13-'14 11.9%.
Intrade has no market for nuclear terrorism events in the U.S., no doubt in part because a prediction market doesn't function when the outcomes in question make the contract unenforceable. That said, most terrorism experts put it at beow 1% over the next decade, athough there is a broad range, and one places it as high as 29%. (Bunn, Matthew. 2006. A Mathematical Model of the Risk of Nuclear Terrorism. Annals of the American Academy of Political and Social Science September: 103-20, and a good summary of multiple opinions here.)
Of the existential risks of liberal democracy, weapons of mass destruction present a less likely but more consequential risk than states' failure to budget sustainably. Whether this reality percolates down from the political heavens to voters that insist on making it a central issue in their political process seems to be, unfortunately, an entirely separate question.