The always excellent "Behind the News" interviewed Yanis Varoufakis a few weeks ago on the crisis in Greece (the second half of the show). Short story: if Greece leaves the Euro, bank runs in other periphery countries on expectations of devaluation (e.g. Ireland), and Germany will leave the Euro so it doesn't have to bail them out; the Euro monetary union will be gone within days. At that point Germany and other surplus countries will have a deep recession as their new currencies appreciate and their exports become expensive.
More at Varoufakis' blog.