From Nature.org (via digg):

In fact, if you leave your computer on 24 hours a day, it could be responsible for releasing up to 1,500 pounds of CO2 into the atmosphere every year. The flying-toaster screen saver is cool…but is it that cool?

But if you have to leave your computer on, here’s a way you can make up for it: By joining a distributed computing network that models the effects of climate change.

Distributed computing networks harness the unused power of thousands of personal computers to perform complicated tasks.

For instance, climateprediction.net is a distributed network run by Oxford University and other partners that helps climate scientists run climate models on networked computers when those computers are on, but are not running at full capacity.

Wow - leaving a computer on to such a degree really does leave a heavy carbon foot-print for something that would just be sitting there otherwise. I say turn it off, but, yeah, if you do leave it on why have your CPU work away on something useful.