In theory it should, of course, be the creditors. You buy an asset (govt bonds), you're implicitly and knowingly shouldering a risk of default. The institutions that bought these assets should have known what they were buying. Caveat Emptor.
Of course in practice you need to ask what the wider implications of default are. Greece _should_ be allowed to default. Absurd austerity budgets are penalising the average Greek whilst minimising the bottom line impact to (arguably wealthy and powerful) European banks. It's no wonder they're pissed. The short-sharp shock of a Greek default will be over in no time. Rolling debt and refinancing terms etc etc is likely to make things worse in the long term.
But of course Italy is different, given the size of the economy. The impact of a default is likely to be far, far wider than a write-down to the creditors. But to be honest I think this worry over contagion is being over-blown. Markets often over-react. It only takes a single hedge-fund these days to move a market - even a big one like Italy debt. It's probably just Bluecrest or Moore selling the market :)