My initial thought is the HDD, but fish out your restore discs and run Apple Hardware Test (extended). Or you could just take it somewhere to be checked out. It should still be under warranty, so that may save you a little time. If they find a problem, they can check the system in right there, order a part, and the whole process is that much quicker.
My iMAc 27 inch 2.93 GHz i7, with OSX 10.6.4, randomly freezes once or twice per day. The mouse pointer can still be moved around the screen, but can not be used to select anything. All applications quit working once the iMac freezes, and the only way to get the iMac to work again is to power down.
This issue has occurred using different applications: Safari, Firefox, MS Entourage, MS Word, etc.
Does anyone have any suggestions?