Gregory Ferenstein takes the sit-stand desk one step further:

The Lifespan TR-1200-DT5 places a square standing desk atop standard-size treadmill (74″). Instead of a large front dashboard, a relatively discrete control panel for speed is attached on the body-facing side of the desk. Speed varies from .4 MPH to 4MPH (about the pace of a light run).

Migrating to the walking desk was relatively easy: I just plopped my laptop and monitor down on the squarish 46-inch desk and got to work. The intuitive interface lets you choose several tracking mechanisms for calorie burn and distance.

The first day I couldn’t walk more than an hour at a time before I felt like I was losing concentration. It also takes some getting used to walking like a Tyrannosaurus rex (arms tucked-in and elbows bent at the keyboard). At first, I would work for an hour walking, and then sit for 30 minutes. The first day I walked about four hours. Now I only rest once a day. It also took a bit to develop the musculature in my upper back to support raised arms for hours on end. This is no longer a problem.

After experimenting with different speeds, I now vary between .8 and 1.2, picking up speed in the late morning/early afternoon to offset the natural fatigue that precedes the morning news rush. Every so often I have to lean on my elbows or straddle the rails to take a break.

Sounds like a great idea.

See also Joyce's writeup about her sit-stand desk: A week with a sit-stand desk

For those who believe in calorie counts:

Actually, it's not so much about calories expended as it is about getting wind and improving posture and general health:

The first week, I dropped 3 pounds (as expected from the 1500 calorie/day deficit) and a whole percentage of body fat (as measured by a Withings scale). Subsequently, my weight and body composition stabilized. But, that’s probably because I have the metabolism of a catatonic gerbil and can’t seem to find any diet that works. Others may have different results.

Additionally, I find myself better equipped to take on San Francisco’s hills and will often walk to a meeting now if its less than 1.5 miles away. Indeed, I’m addicted to being upright. One day, when the subway broke down on my way to the gym, I just ran to class from the Mission to downtown SOMA (roughly 2.1 miles)–a surprise to me since I don’t think I’ve ever run two miles straight.

The problem I face now is loathing to work sitting down. While traveling, I’ll look for coffee shops that have a window facing bar so I can at least stand during work.

I definitely also have a harder time sitting for hours.  In my case the main issue is that I don't like to drive or fly long distances any more.

It's probably even hard for you to sit for a movie or sporting event these days.

Why is so much of American life centered around sitting?

The price of walking desks is still rather high.

Lifehacker has some DIY suggestions: 

http://lifehacker.com/5729625/a-beginners-guide-to-setting-up-a-treadmill-desk