Hey JJ, I recall you being there at launch for the new Mac Pros, just as I was. I had purchased the 3.33 Hex core with an ACD 27", but sold the Pro for a new iMac i5 and I'm very happy that I did.
I also picked up the new 11.6" Air as well and the iMac + Air combo is working great for me. I kept the 27" though as it's just such a beautiful display. And actually, my wife has been borrowing my Air as of late because her HD died on her MBP, so she has it currently plugged in to the 27" ACD.
I can tell you that HD playback isn't as smooth as my iMac or the Mac Pro when pushing to the 27" ACD. I should mention that I went for the ultimate MBA configuration as well. So I second Nonarkitten's comments with the stuttering issues (even on the 1.6GHz) and the iMac route as well. Or at least a Mac Mini even? But if you're hoping for a laptop, "take on the go" setup, the Air won't give 100% perfect playback on the 27" ACD.
That said though? I still love my Air. That little machine still kicks some @$$ in other areas. Love the flash drive