The main reasons why the US$ will not fail(at least not this year) are in my mind the following:

1) it will be political suicide for any major politician to oppose the US economy and financial markets since that clearly will affect everyone(as we have seen)

2) everybody is owed US$ by the US, all their investments will drop like a stone if the US$ will fail

3) there are no-one right now(except gold) that can take over the leadership. Europe has almost the same problems as the US and China has also many issues

The reality is that US and Europe have lived beyond their means for decades on borrowed money and borrowed time. The question is for how long will the producing(China, India, Mexico, etc) world continue to subsidize our standard of living. I think for a while yet because even they have improved their standard of living during this period, so the alternative is pretty grim also for them