I'd personally put the probability of a country abandoning the Euro this year at <5%. I think the major European powers (e.g. Germany and France) are still committed enough to the monetary union to try to make things work out. However, if corrective action fails or is rejected by the voters of southern Europe, then I think we'll see a greater willingness to abandon the Euro by all parties.

EDIT: This raises the related question of, "What is the probability that Greece, Spain, Portugal and Ireland will agree to and implement sufficient austerity measures to prevent a breakup of the Euro?"