We did it in early 2006 http://discourse.weather-watch.com/t/16460, it was a pain to get everyone signed up and running properly, many of us had multiple PC’s running, and then all our efforts were pretty much a waste of time because the program was faulty http://climateprediction.net/board/viewtopic.php?t=4697
Be aware that there’s really no such thing as spare CPU cycles, the harder your CPU is running, the more power you will use 