The euro-zone bailout of Greece is, in good part, a bailout of European banks. In France and Germany alone, banks hold some $90 billion worth of public and private Greek debt. The European Central Bank also holds Greek government debt, and the fear is that if Greece defaults, cascading losses could threaten all of Europe.
Are American banks also vulnerable? No one is sure. They are not big lenders to Greece, but they are big players in the derivatives markets. If Greece defaulted, a European bank holding a credit-default swap on Greek debt from an American bank would be entitled to a payout from that bank.
Credit-default swaps are the kind of derivatives that were behind the blowup of the American International Group and the near meltdown that followed in the global financial system.