My Holy Grail:
--The belt would be at least 18" wide (preferably 20"), and be padded.
--The motor would be quiet and designed for long distances at slow speeds (which wear a normal treadmill motor out).
--The treadmill could go up to 4 miles an hour, because even though I can't type at anything above 2 mph, sometimes at the end of a workday I just want to watch some shows on hulu, and for that, I can do a very brisk walk.
--The treadmill could incline.
--The controls would be connected to the treadmill by a wire, and could be mounted far to the side, so that they wouldn't get in the way of a keyboard tray mounted underneath the desk.
--The height of the desk would, of course, be adjustable--but electronic adjustability is not necessary, as how often will I be moving it?
--And, of course, it would be cheap. By "cheap" I mean "in the neighborhood of $2000 for the complete package."
None of the premade ones have all of the features; few even have most. :(