I didn't see this on here, so I thought I'd post it just so people are informed of some trouble the headphone jack is causing on the new iMacs when under Windows: http://discussions.apple.com/thread.jspa?threadID=2239832&tstart=0 Basically, using the headphone jack fully utilises one core. I wondered why my iMac was heating up when I wasn't even running a game in Windows. If you want to test it for yourself, load up Task Manager, plug some headphones in for a few seconds and watch.
Mine has the same problem (i5 2.66 GHz). 0% CPU when no headphone jack is plugged in, but 30-60% when it is.
I fixed this by slightly pulling out the audio cable, as described on the thread you linked to. Thanks!