I've experimented a bit with a treadmill+laptop, but the first thing I tested was my spaced repetition performance, which went down while using the treadmill: http://www.gwern.net/Treadmill#treadmill-effect-on-spaced-re... So I'm not sure if I want to continue trying to use it or reserve the treadmill for winter when I can't take my usual daily walks.

Maybe some kind of AR workspace setup based on the new Samsung Gear VR would be feasible? Would look pretty crazy, and still might be a bit of a safety hazard, but it might work. I love standing desks but the closest I've been to a walking desk is trying to type while rolling the laptop cart through the datacenter. It kind of works.

I use one built out of an old treadmill and a plywood desk I made. Works fine enough though the motor overheats at about an hour, even when I use silicone spray on the track and a fan blowing on the motor. Treadmills aren't usually made to run for an hour at very slow speeds (1 MPH or less).