I've had a treadmill desk for about a year now. I started with a standing desk but found my legs got too stiff being still so I hacked up a $150 used treadmill. This has completely changed my work life. I had horrible back problems which made me question if I could continue as a developer. I had tried different chairs, desks, postures and had some success with software timers which forced me to move around every so often. After a few weeks of the treadmill desk my back was feeling better. After a few months all problems were gone.
I usually walk between 1-2 mph and have no problem typing and concentrating. The main drawback is that my work station is the treadmill desk and I can't sit. When I get tired I take a laptop to the couch (I work from home) but I'd rather be able to stay in one place were my good setup is. There are some desks which raise and lower mechanically which I plan to get at some point. http://www.treaddesk.com/">This company has some but they are expensive.
With the addition of an arduino I also geek out with data from the treadmill.