A Cornell University study:
So the key is to build movement variety into the normal workday.
Very interesting to read, the key thing that the report says is that you need to move around. While I love standing at work, I also move frequently while at work and sit for long periods to and from work (and after work hours). Ideally I would like an adjustable desk, but I think not having an adjustable desk actually makes me move quite a bit more.