This is a picture of the laptop desk I created for my treadmill. It involved a rectangle of 3/8" plywood (you can often get scrap odd cuts wherever they sell plywood), and several feet of 1x2 trim cut to create a hinged bar that props the desktop using the treadmil handles. It's attached with two holes at the top of the desk and a bungee cord wrapped around the frame. A 15" strip of trim at the front of the desktop keeps the lappie happy and untrampled. If I did it again, I'd extend the trim all the way across the desk.
I find it really easy to plop my laptop and check email or RSS reader while I walk absolutely nowhere for an hour. The materials cost less than $20. (I got the treadmill on Craigslist for $35). The sense of satisfaction in exercising while working? Priceless.