I'll admit, I'm having a lot of difficulty resisting the charms of the top-end 27". I currently have a 2GHz 1st-gen Mac Pro, and it would be a nice bump up in speed and capability, but there are two main things that might keep me from buying the iMac:
1) No SSD option. I'm currently booting both my Mac Pro and my unibody MacBook from SSDs, and am loathe to go back to platter-based storage for the OS/Applications.
2) Nothing better than FW800 for external storage. I currently run a 3TB RAID for my data on the Mac Pro, and could deal with moving that to an external box of some sort, but I'd take a tremendous performance hit with anything less than eSATA.
In the end, I will probably wait until Spring and see what the next revision of the Mac Pro and MacBook Pro offer. If the top-end 17" MBP gets a quad-core processor, more RAM expandability, and keeps the ExpressCard slot, it might make more sense than the iMac, since I can put two hard drives in it (including and SSD system drive) and get an eSATA adapter for more external storage.
If the new iMac had either an SSD option (just a small panel to allow installation of a 2.5" drive would suffice) or eSATA, I might bite the bullet for the extra performance over my Mac Pro, but without either, I'm still on the fence.