Eh, doubt it. A lot about ventilation is having a directed flow of air. With the iMac, cold air comes in through the bottom and it travels all the way up and out. Pretty much ideal. If you put a bunch of holes in the back, everything becomes much more complicated there's the possibility of the air flow being "short circuited" in certain areas, maybe some components wouldn't be cooled as effectively, etc.
It does bother me how hot the current iMacs get, especially the 27" under load. I'm sure Apple is trying to save money on heatsinks and trying to keep the computer quiet by running the fans slow but it is amusing to think of different potential cooling systems. Since the iMac is updated so infrequently (grr) it seems like they could make custom heat pipes that run to a big heatsink along the top of the case under the ventilation slot. It would be pretty neat...
iMac's designed to get hot. The aluminium works as a heatsink.