I don't have a treadmill desk at work, but I do have my desk set up so that I can stand at my computer as I work. I do this from 4 to 7 hours per workday (31 hours total last week). I think that being active in this way has been pretty critical in my weight-loss. Even standing and not walking burns twice the calories that sitting down does, and over the course of a day...a week...a month...a year...two years, for me now!

it really adds up. And, doing all this NEAT (Non-Exercise Activity Thermogenesis) means that I can eat a lot more calories and still lose weight at a good pace.