On a pre-iSight iMac, you could simply take of the back cover (wouldn't void the warranty) and have a look at the hinge, if something's wrong. Unfortunately, Apple decided to entirely change the built up of the iMacs with iSight (G5 and Core Duo) so that you can't attain the hinge without taking out everything. And, opening it would simply void your warranty...
I would bring it to an Apple repair center, and have them check it.