The founders of Instructables seem to have a reasonably good treadmill desk + code setup (though I personally couldn't tolerate his monitor configuration).
I read this and went to craigslist just to see if it was as cheap as they said. in chicago, there dozens of treadmills posted in the last 3 days and easily ten or so at or less than $100.
http://www.instructables.com/id/Treadmill-Desk/