The probability of a default is low but clearly rising. The decision by Standard & Poor's, the ratings agency, to downgrade Greek sovereign debt and to put Spanish and Irish debt on watch seriously rattled investors last week, for good reason. If the financial crisis has taught us one thing, it is to take perceived tail-risks more seriously.
Before I answer the question, it is best to consider what would not happen. For a start, the eurozone would not fall apart. A government about to default would be mad to leave the eurozone. It would mean that, in addition to a debt crisis, the country would also face a currency and banking crisis. Bank customers would simply send their euros to a foreign bank to avoid a forced conversion into a new domestic currency.
So if a default were to happen, it would almost certainly happen within a eurozone that remained intact. If you put your mind to it, it is quite difficult, even in theory, to think of a circumstance in which the eurozone would blow apart. One theoretical possibility would be for the European Central Bank to generate massive inflation, prompting Germany to leave in disgust – not exactly the most likely scenario right now.