My prediction is that america right now is too big and important to fail. There is too much global vestment in america being strong and stable. However the result of the recent financial meltdown have brought it to everyone's attention that there is perhaps too much reliance on america. Countries will seek to strengthen their local trading blocs and devest in america. This will gradually erode america clout and will force a serious revision of their spending habits, especially on military. This will take a decade or two, and trickle down to the domestic level where there will be a painful (much doomsaying), gradual, but needed revolution in personal spending, debt, and responibility.
This responsibility will re-enpower the middle class and america as a nation will shift into a more cooperative role. It will be a bit of a golden age for them, if a humble one. At no point will the country risk actual human disaster like widescale famine but tensions will spark over energy consumption and lack of individual purchasing power, unless there's a breakthrough in power generation.