well now, heres an interesting comparison. The BBC just ran a program discussing US debt. Which is out of control. The US will not do what is the simple requirement, increase taxes and cut spending. It is of course true that this is a bad time for it, but the US has failed systematically to do so through good and bad. They said that in the Clinton years the US could have reached zero debt. Since then taxes have been systematically cut, expenditure on home security has gone through the roof, there have been some very expensive wars, the small matter of trying to fix a problem over some bankers selling duff securities. On the expenditure side, there are vast unspoken commitments to welfare, health and pensions. The BBC seemed to reckon matters are not yet desperate but its getting close and no sign of a fix yet.
So, what happens if Greece defaults and it starts to cost the US? Might a Greek (et al) default actually have the effect of destabilising US finances enough that it is the dollar which collapses?
As I said before, what counts as a good currency is the best amongst some bad choices.