Sitting is not necessarily bad--just sitting the way we do in our modern culture is particularly bad. I already recommended it somewhere here, but look up the Authors@Google talk by Esther Gokhale. She looks at the posture of cultures around the world who experience almost zero back pain. She makes some good recommendations on how to sit.

I do have a standing desk though. I just put my computer monitor onto the top shelf of my desk, which is at my eye level. I placed the keyboard on a box on top of the desk surface. Generally I try to do a lot of my listening/learning while walking outside now, with my iPod Touch. There's a lot of evidence that some people, especially boys, learn better while moving. Many of the great minds in history would discuss their ideas while walking. It makes sense evolutionarily because much of our learning was about our environment.