The fellow who created PinPoint, the software I use for that cool red arrow pointer in my BuildAModule screencasts, also works on a treadmill desk, but his is mechanical. I always thought that style wouldn't work so well because with the electric version, it requires a reaction not to fall off, whereas if you get lost in thought on a mechanical one, I would think your feet would just stop moving.
Anyway, Renaud says it works pretty well, but with a slight modification. He added a hanger as a waist-bar so he can push himself with his stomach, which I would think could add some additional physical benefits to the process.
Here's some pics: