Oh, there's a temp sensor in there somewhere. The Core Duo just runs cool enough relative to the early G5s that you don't hear the fans much.
Finding the sensor and cutting the wire (or leaving it disconnected) would of course force the fans on full blast, but that's not a useful soltion and there is nothing in software that I know of.
Really, it's almost certainly fine--it just runs warm, as pretty much every computer does (heck my ACD gets VERY toasty, and it doesn't even have a fan--sure it's not just the screen getting warm?).
But here's something you can try if you're freaking out about it: On the Intel Minis there is a way to trick the fans into running full blast all the time. If you disconnect the power (and everything else) from the computer, then hold down the power button while plugging the power back in, when you hook everything else up and boot the fans will be stuck on full blast.
This MIGHT work on an Intel iMac, too, though I haven't tried it. It shouldn't hurt anything--you're just confusing the firmware control of the fans--but it's very noisy.