At some stage there will be a default in Greece. There are a number of ways this could happen. The most likely of these will involve a Greek retreat from the Euro, the reintegration of a local currency and the element of surprise (sort-of). Whatever the case, it will send another wave of shocks around the banking world and make consumers even more cautious, but more importantly, it will become a model for those who follow (the rest of the PIIGS).
The most interesting thing (for those who don’t have their capital tied up in these countries and who have limited exposure) is the practical day-to-day of what a default looks like. If Greece reinvents its currency overnight, it will make deposits in Euros in Greece worthless. Canny Greeks will have their hard earned stashed away in cash (or will be in the process of withdrawing it from the bank) and will be looking to physically take this cash somewhere else to deposit or exchange.
For an historical perspective take a look at the Hungarian hyperinflation of the pengő in 1946 and the Argentineans in 1999