Now the reason this is a hint, is because it almost makes no sense and when I posted my own answer on the Apple discussion boards, I got a lot of reponses from people saying they hadn't ever come across it.
It seems the rules for reseting the power management unit for the iMac G5s is the same as that of the PowerMac G5 machines. In my case, the late 2004 G5 processor, that simply involves removing the main power cord for a few minutes and plugging it back in. Once I did that, my fans returned to normal speed.
But a hint of advice ... if you are having a problem, search for an answer to your processor and not your machine! It may save you lots of time.

