If the Greeks default, various European Union countries, such as Germany, Italy, Spain and France will lose billions of Euros. Germany alone will lose some 56 billion, which can crush the German economy. Economies all over the Europe will be affected as a result of this, as most of them are dependent on Germany.
It still seems likely to me that the ECB/IMF will fold at the last minute because the consequences of a default far out way the costs of restructuring Greek debt (read: write some of it off).
I'm sure there are a few busy calculators this weekend calculating the exact cost of each scenario.