Ahh...that pesky sky is falling again.
I don't think America is going to fall. If we take off the whackoid eyeglasses and take a look in more sensible manner, here is what can happend:
Slight production downturn, higher fuel prices, RE market coming down (not crashing, just adjusting to reality), interest rates going up, dollar value depletion due to foreign (un)balance, standard of living going down...
That will hold up for few years, then people will respond to it, get down to bussines with (hopefully) new leaders and everything will be relativly back to normal after 10-15 years except being the economic superpower. That part might be hard as new players are emerging.
Mad Max scenario is unlikely. I don't think that even terrorists are stupid enough to try a WMA strike as that would trigger some very very unpleasent paving of ME region which would hit both those who were guilty and those who were not.