I "made" a very crude treadmill desk a few months ago and I love it. I can comfortably go 2.0-2.5mph while programming and I try to use it for 2-3 hours on most days. Here's a photo:
It's a cheapie treadmill that seems to have been discontinued. The model is Proform Crosswalk 400E.
Based on my experience, I'm guessing any kind of treadmill would work as long as it has a regular sized belt. You want to have enough space behind the "desk", otherwise your feel will be falling off the back of the treadmill when you walk.
http://leopolovets.com/img/treadmill_desk.jpg
Basically it's four five-dollar plastic bins on each side of a treadmill, and then a ~2'x4' plank on top. It looks awful but works very well.