Here is an idea:
On the dell website you can custom build a laptop, pretty much everything - processor, ram, dvd drive, keyboard / backlit, wireless etc. I would say from month to month what you can put in the laptop completely changes because new hardware is constantly being released - obviously. However the laptop model itself doesn't particularly change (thinking dell studio)
So why do apple not do this with the imac? There is no need for a huge press release or event. Anyone who has ever built a pc knows how easy it is to just pop in a new processor. I'm sure apple employs systems builders for what little custom options it has so popping in a quad core and blu ray should be no problem. Then apple would be with the times in terms of hardware.
Just a thought