Well there's been a year and a half of legitimised money laundering where banks have traded their greek debt for euro debt via the ECB, but the french, german and UK banks (mainly RBS) still hold a lot of the debt. Of course if we have a default then the CDS trades come into play and that drags in the swiss and american banks. Following the failure of lehmans banks have been busy hedging their CDS exposures so it's difficult to see one big bank taking the whole hit and going under - bail out for a couple of the french banks probably. And when the tide goes out we'll see who's swimming naked - ie there's always fraud in the system, and someone will be caught with their pants down.
It does however hit capitalisation of the banks and makes debt more expensive for everyone else.