I took my iMac back to the store today, for the third time in the nine months I've owned it. The current problem is that the fans are on constantly, and the thing is twice as loud as my refrigerator. It's completely silent just after it boots, but after exactly 5 minutes the fans start ramping up, and by 10 minutes, they're on in "windtunnel mode". This is true even if the machine is just sitting at the first page of the installation DVD. I've reset the SMU, PRAM, and NVRAM, so they're giving me a new logic board, again.
And when I was dropping it off today, the guy in front of me was complaining about the exact same problem! If I heard him right, he had also just had his logic board replaced. And the tech was looking like he'd never heard of this fan thing before.
How about that.