Why would they wait for a MacWorld? They didn't announce the iBook G4 there. Also, there's no reason why a G5 iMac would wait until after the Powerbook; the iMac has a much better cooling capability. If they were making radical changes to the shape, that might wait for a MacWorld, but frankly, how likely is that?
Nah, none of those reasons hold any water. Now that the Xserve is using 90nm G5's, there should be PowerMac updates next week, and there's no technical reason that iMac G5's couldn't follow within a month or so.
The next step for us rumor-mongers is to start watching inventory levels of iMac G4's. When they start becoming very hard to find, that's when to start looking for the iMac G5. At present, iMac G4's are selling well and in plentiful supply, so it won't be very soon.