Here's an idea that only goes half way: Furniture maker and D6 sponsor Steelcase was showing off its Walkstation product. It's a stand-up desk integrated into a treadmill. So now you can take a walk while you're working.
I tried it and it's not as awkward as you might think. You have to get the speed on the treadmill just right, so you can walk without thinking, but once you do it is actually possible to read and type while walking.
The device is thousands of dollars, steep for the average worker. I fully expect the Walkstation desk to show up on CSI soon, as metaphor for wretched executive excess.
And here's why the device doesn't go far enough: It lets you expend your energy but doesn't do anything with it. In an era of emerging environmental consciousness, I don't know how it's legal to build exercise equipment that doesn't feed the energy that people expend working out back into the power grid, or at least back into the equipment itself. I'm serious.
See also: More Walkstation coverage on Crave.