I'd put the odds heavily in Contador's favor. I don't think, however, the odds are as bad for Andy as most people think. We have seen Alberto implode spectacularly from time to time, most notably in last year's Paris-Nice.
Of course, 1 bad day can happen to anyone: we've seen Armstrong lose big chunks of time during his 7-victory run, Landis had a bad day during his 2006 run, etc.