Parade Magazine published this piece by A. J. Jacobs and his introduction to standing up while he works. I've been using a stand up desk for quite awhile now, and don't have a sit-down desk in my office.
I do, however, have a small, round conference table with four chairs, so when people come in, I may offer them a place to sit (or not!).
I also have a small sofa, the main purpose of which is to give me a place to grab a short nap in the afternoon.