The reason updates are slowing down is not because Apple is an iPod company and it has forsaken the Mac. The G4 is an older technology that is reaching the end of its life in personal computers. There is only so far you can push any given technology and aside from dual core G4's, they have been pushed about as far as they will go. G5's on the other hand are a very new technology and IBM, like AMD and Intel, had many more problems than were expected when they transitioned to 90nm technology. Now that yields are increasing, we will likely see more and faster G5's as the year goes on.
Apple does not want to put a G5 in the mini just yet and the Powerbook isn't quite ready for a G5 and even moreso the iBook so these product lines are stuck with the G4. The Powermac has the fastest available G5's currently (so far as we know for sure) and it can't be updated until they are able to push past the 2.5GHz barrier without the use of liquid nitrogen or a wind farm. Who knows the status of the eMac? That leaves the iMac.
Why would Apple not update its most renown computer line when it has the technology to do so readily available. Sales of its professional lines are already in the toilet so why send the iMac with them?