I think AC is beatable, but not only by AS, there are others out there with the potential to beat him. But I can't see him losing the 2010 TDF, given the mountainous parcours. We know he is pretty good at TT now, but we also know that he does struggle when the TT distance exceeds 40km (in fact he lost a lot of time in the last 12k of the TDF time trial), however over a 50km TT I believe he will still outdo Andy S.
The only way I can see AC losing the TDF is if they repeat the parcours used in 2007 TDF - lots of mountains with descent finishes leading to lots of attacking by Rass and Soler (and the new kids on the block) and two long FLAT time trials. That said, under that parcours, it would not be Andy S beating AC, it would be someone like Evans, Levi, Wiggins or LA (if he learns to TT again).
I think it is clear that AC is talented, 4 GTs, underdone in the Giro, pressure in the Vuelta from Levi, lots of pressure in the TDF from Astana, and he still provailed in leaps and bounds, but he has yet to do this without the top team support (yes I know Astana didn't support him this year - but the TTT sure did). If he is on Astana next year, it will be very interesting to see. There will be 6-8 contenders on 6-8 teams which should make for very interesting racing, and all the pressure is on AC to defend his title. He is yet to be tested with this many contenders for a TDF title and not the strongest team