I beleive you can. Here's an article and video with a TB iMac connected to two dell 30" dvi displays. If it can handle those it should handle one apple 30"!
All you need is a mini-displayport to dual link DVI adapter to plug into the back of the iMac.
And here's the confirmation. The Apple 21.5" and 27" iMac specs. In the "Video Support and Camera" section it states:
Simultaneously supports full native resolution on the built-in display and up to a 30-inch display (2560 by 1600 pixels) on an external display
(bold emphasis mine)