I was wondering about this. I've been trialing a standing desk - the best part is if you are thinking about a problem you tend to walk, where as if you're sitting down you just tilt your head to the side.

But after 4 hours or so I find I get a sore back. I've been "mixin it up"... standing for an hour, sitting for a couple of hours. Or sitting in the morning, standing after lunch... so far it feels pretty good, but I'm keeping some (quite subjective) stats to check out how it goes in the longer term.