The fans (there are two) normally idle at 2000 rpm, Geoff. Under stress they can run at up to 6000RPM. At 2000 they are pretty well inaudible - quieter than your HD idling. At 6000rpm you will certainly know they are there.
They don't kick in to faster mode seriously, though until the temps get quite high. Even at around 75ºC CPU temp you will find things are pretty quiet. Head to the mid 80ºs C and they will be ramping up, but still not flat out. It is remarkable how effective even a small increase in fan speed at such temps is at getting the temps stabilised though, and it is not a good thing to have fans operating aggressively unnecessarily as you don't want sudden temp changes .
The CPU can run at up to 100ºC or more so don't worry about this. I've never seen mine anything like even 90ºC by the way, even under extreme load, and it usually runs in the 40º's and 50º's C.
Games and video rendering are the two things that tend to send the temps up and the fans revving. If you don't do much of either of these you may never notice them at all.
The best "system monitor" software (for both your iMac and your MBP) is Marcel Bresink's "Hardware Monitor" -
http://www.bresink.com/osx/HardwareMonitor.html
If you want a good way of checking temps, fan speeds etc it is well worth the 7 Euro fee (there is a free, but function limited, trial version, too) . It gives a very wide range of information, is very accurate, and provides "history" graphs that let you look at things like fan speeds, temps and power loads over time, and to compare the behaviour of each, etc etc.
If 7 Euros is too much, then iStat Pro is worth a look, but it doesn't have all the bells and whistles of Hardware Monitor and has produced the occasional "wake from sleep" glitch in the past.
Cheers
Rod
