hi, i found a good solution to keep my iMac 24" 2008 cool:
make a minor modification to the chassis:
1. open the mac (iFixit has clear instructions on how to do disassemble the iMac, e.g.
http://www.ifixit.com/Device/iMac_Intel_24%22_EMC_2134_and_2211)
1. cut away the upper plastic rim of the venting strip on the back of your iMac.
You can do this cleanly from the outside of the chassis by letting a 2 mm drill glide on the metal grille, and slowly letting the dril cut away a line in the plastic.
2. strip away the aluminium adhesive strip that blocks the large rectangular openings in the upper exhaust grille (peel the inox-contact pads off the strip and put them back on the metal grille)
3. drill a lot of venting holes in the plastic back. You can do this cleanly by letting your drill guide by the holes in the metal upper grille. Drill from the inside of the chassis to the outside. Drill 3 rows of holes this way where you can reach through the grille. Especially the region above the powersupply should be well vented. This is where most of the heat is generated in my case.
You can see my result in attached picture here, and I am most pleased with it.
With what I know now I would now make three rows of holes instead of two.
