According to a recent report by Fitch, as of February, 44.3 percent of prime money market funds in the United States were invested in the short-term debt of European banks.
There is more detail here; fortunately, not all of them have heavy exposure to Greece. You will recall also that “runs on money market funds” were one problem which regulators have yet to address in a satisfactory manner. Here is another claim, with an uncertain degree of verification:
It will be American banks and insurance companies that will have to make the lion’s share of default insurance payments to European institutions if Greece fails…if one includes credit default exposure, American exposure to Greece increases from $7.3 billion to $41.4 billion.
It still remains the case that without contagion effects these losses can be handled.