According to this Wall Street Journal report, call-center workers given “stand-capable” desks were 46% more productive than their peers who remained sitting.  This astounding improvement in output is attributed to the benefits of moving around.

“We hope this work will show companies that although there might be some costs involved in providing stand-capable workstations, increased employee productivity over time will more than offset these initial expenses.”

-Mark Benden, Director of the Texas A&M Ergonomics Center

P.S. It seems to me from what I gather off the internet that sitting or standing all day at work may not be as healthy as varying positions, e.g., see this essay from 538 blog.