Seems Apple was going to announce a new iMac at WWDC, but a parts shortage kept them from being able to do so.

My guess: G5 chip and a case redesign. They'd need the case redesign for the G5 anyway at this point, and if the G5 was the only reason for a parts shortage then they could have fairly easily used a faster G4 and waited one more generation. No, my guess is that the new cases are what's really holding it back; probably a shortage in one of the parts or something.