A lot of commentators, particularly Americans, tend to think a default on debt repayments means Greece would have to leave the Euro. This is simply not true a default on its euro denominated debt will hit creditors but not automatically mean Greece leaves the Euro. In fact I think there is every reason to believe Greece could actually stay in the Euro in the event of a default as it has a primary fiscal deficit of only around 1% meaning that its taxes are roughly the same as its Government expenditure. It is the interest payments on its National Debt that make the deficit much bigger but these payments cease in a default scenario.
In my view the most likely scenario remain that Greece stays in the Euro, undertakes reforms sufficient to "rollover" its debt and eventually we get a further debt reduction package in couple of years time. It is not in the interests of the creditors or Greece that it leaves the Euro and goes back to a new drachma. The loss of payments to the creditors and the potential economic shock of a country leaving the Euro would put immediate pressure on Ireland, Portugal , Italy and Spain who would all be vulnerable to speculation about possible exits from the Euro. The Guardian has a nice article today discussing the benefits/costs of Greece exiting the Euro LINK1 There is a nice schedule of interest and principal repayments by the Greeks to its creditors here LINK2