They've had DiY instructions on these
many times on Lifehacker. Hate to burst anyone's bubble but this isn't exactly a new idea.
There is also a company, IIRC, that makes a treadmill to work with a cube/desk setup.
If I can find it I'll post the link to the article someone wrote about working this way - titled "Written at 1 Mile per Hour" or something like that.
Again, kudos to this guy on building his treadmill desk and to the Reg for sharing it. For us poor souls tied to their laptops for 10+ hours a day this is really a good way for us to get a little healthier.