It's never been out of the question. In the years that they have been kicking the can down the road, all of the EU financial institutions have been preparing for this exact thing. You can expect Greece to exit the euro with little to no effect. The euro will of course rise, because the caliber of the remaining countries are much stronger.
The real issue is contagion. The EU doesn't want Italy or Spain to go down as well, because the markets/bond vigilantes love to pile on when there is blood in the water. Look at what happened during the Asian currency crisis, it was mass contagion throughout Asia that caused the subsequent problem.