Wow, the infamous 4850 strikes again.

It caused delays last time around as well!
It also had a pretty bad issue where it would make iMacs freeze (until they patched it).
BIG EDIT:
You are actually 100 percent CORRECT. They delay is not for the quad core, it is definitely for the 4850 card. If you go on the Apple Store, and you select the base (core 2 duo) 27 inch iMac, and then upgrade the video card to the 4850, then the core 2 duo iMac immediately goes from 1-3 days ship time to november. This does not happen if you customize anything else in the iMac.