Here's an interesting article from the New York Times about standing while working.
Over the last 2-3 months, this topic as been showing up quite frequently in fitness magazines, blogs, and tweets.
A study, with reference I can not find at this time, showed that a large percentage of people who suffer and or died from obesity related illnesses sat at a desk to work (as opposed to stood or were mobile). Be careful not to read this as people who sit all day long are more prone to death than others as this is not the result of the study; albeit a very close resemblance.
Also, to note, the offices at Stack Exchange are equipped with height adjustable desks.
I recently tried standing while working for about a week. It was a nice change in posture, but I don't recommend doing it all day, every day. The first day, I stood for about 6 hours. By the end of the day, I could feel the fatigue in my legs and joints. The next day it was better. Currently, I have the option of sitting or standing and will switch to standing when I feel my sitting posture is bad.