It's strange that distributed computing has not leapt onto this. Granted, it's a narrow nieche, with the program running only for a few minutes and with a limited size, but there are still plenty of cases where it could work. I've only found This proof of concept of distributed flash computing. This example require some user interaction and fills a whole screen, but there seems to be no reason why it couldn't run as a banner ad.
Why isn't this more widespread?