It will only become a threat to the US economy if it is perceived to be a threat to the US economy. Currently out GDP to Debt ratio is far better than any of the European Counties in crisis and US banks don’t seem to be too deeply entrenched in the crisis. This means we ‘should’ be able to avoid problems. However, (a nice way of saying but) at some point US banks are going to have to deal with overvalued mortgage assets; the foreclosed properties are still on the books at clearly inflated prices. This has allowed banks to post profits over the last few quarters where profits are not quite as clear. US banks may use the weakness in Europe as a catalyst to right size their own liabilities.
Investors may react to the fiscal problems in Europe by demanding a more conservative debt structure. This could force the US into a perceived debt crisis.
Personally I think we are seeing this now. The multi-trillion dollar austerity measures being pushed by both parties are a reaction to perception not reality. This is not to say we don’t need to solve the problems that are being addressed I just think we need to move from our glutinous diet to a healthy one rather than subscribing to fiscal anorexia.