I thought of this as well, but more along the lines of protein folding. I think finding a cure for one of those diseases would be more than enough for most people to justify the power consumption.
Folding@Home, and the BOINC network over at Berkeley have done a great job at this already. http://boinc.berkeley.edu
There's still an immense opportunity to tap unused cycles if you give people another reason to donate their device time. Unfortunately, altruistic purposes don't always appeal to the masses :-)
