I'm not sure about this one. It will depend on how agressively Apple prices these.
I have a 20" iMac, and with my edu discount (at the time), I paid about $2500 (Canadian) for it with 2GB RAM (from Apple...was cheaper than anywhere else, believe it or not!), 256MB VRAM, and a 250GB HD.
For about $350-450, I can get a decent (not great, but decent) 20" widescreen display and hook it up to my iMac, giving me 3360 x 1050 resolution, which is plenty wide enough to display 36+ faders in Logic, plus some other apps (Live, Reason) that I use with it. A 23" display would only give me 1920 x 1200.
The point is: depending on how Apple prices it, I can get the 20" plus an external monitor and get more space for less money.
I can see this working if (a) the 23" isn't substantially more expensive than the 20" AND (b) there are a few other things (i.e. bigger HD, more RAM, faster/better vid card) to distinguish the machines. This is the precise reason I didn't get the 500GB drive in my iMac...I went up to 250, but I could make up the additional 250GB over Firewire for a lot less.
Either way, these should be sweet. I love my iMac (it does EVERYTHING I need today) and I don't anticipate upgrading for a number of years (well, maybe I'll slap in a Merom after the Applecare expires!)
Anyway, just my two cents.