The USADA had George Hincapie -- the guy who was Armstrong's domestique through all seven Tour wins -- willing to testify against him. That means a lot. In any event, Armstrong's decision is probably more due to the real Feds now wanting to get a piece of him due to mounting evidence that he was essentially the ringleader of a distribution operation.
The kind of numbers that Tour riders were putting up until just a few years ago are now believed to be physiologically impossible to do without doping. There's a legitimate reason cycling has, quite literally, slowed down since the '90s -- people are climbing Alpe d'Huez about 3-4 minutes slower than a decade ago, and that's an absolute eternity in cycling (and just over one climb) ... it's not an illogical theory that this is due to the sport slowly being cleaned up.
USADA claims it has 38 tests from 2009 - 2010 that show evidence of blood doping, plus the 2001 EPO test. I don't see a problem, here -- Armstrong shouldn't get a free pass just because he got millions of people to wear his bracelets, and with him being such a stubborn guy, I don't think he'd make this decision without believing that they finally had him dead to rights.