I run the distributed.net client at priority 8 and after about 2-3 minutes of 100% processor usage, the fan kicks on. I've not used a temperature monitoring program to see what it's looking like, but I would venture to say about 150F is the kick-on point. I have the client set to shut off at 160F and it doesn't shut off, but it did shut off when I had it set to 150F. It's probably somewhere right in there.