iMac Fan Trouble Solved
I had the same trouble with my iMac. I bought it in Aug 07 and the first time the fan went gang busters was in Nov. It did it on and off until March 08. Some of the problem seemed to be related to how long I held the startup button to turn on the computer. Turning off the computer and holding the Command/Opt/P/R keys on start-up, until the second chime, was the only way I could get a month or so without the fan giving me trouble. Three quick clicks of the button to turn it on worked best. If I held it, the fan would go on.
I also made sure to get the latest EFI Firmware Update 1.3 in July which someone told me might solve the problem. It hasn't happened since.