I don't care what you guys say, but I support the no-grunting rule.

It's not like you can't make any noise, but it's an aspect of controlled-breathing when you lift. Unless you are benching 405 and you are exerting all your energy into the last push, I can understand screaming "OHHHHHHHHH." Then again, if you are lifting that much, you don't belong at a regular average joe gym.
Otherwise you would have no reason too. At the regular gym-goer level it's just exaguration. It's natural to grunt, but not natural to scream enough that I can their whimpers through my full-blasted ipod.