I am interested as well.
I currently have a 27" ATD that I use for my MBA. My MBA is my "corporate machine" and the only one that attaches to my corporate network. Attaching the ATD gives me the ability to affectively work from home whenever possible.
My new iMac is on it's way... and my first iMac with TB. It will sit imediately next to my ATD so I will be able to "dual use" my ATD. I'll connect it to my new iMac when I have large projects such as Aperture.
I suspect the performance will be fine... but I do not know for sure. I do look forward to having the dual screen setup on my new iMac.
/Jim