This graph from Paul Swartz at the Council of Foreign Relations shows the time structure of Greek default probabilities for three different dates: The market oracles thus that default will occur sometime around 24 months from now… 

These probabilities are based on risk spreads between German (aka "risk free") and Greek bonds. The IMF, does not seem to have a subscription to the same data. Greek debt default is unlikely according to IMF… Debt default by an advanced economy such as Greece is “unnecessary, undesirable and unlikely”, according to an IMF paper released yesterday.

This runs against market sentiment according to which  Greece will eventually restructure its debts, writes the FT. I guess the story is that "once Greece has cut its deficit to zero, it will not need any new borrowing to finance its budget. Examples in the past 20 years, like Belgium in 1984 or Italy in 1991,showed that when these advanced economies cut their deficit none of went on to default."  Time will tell…