[Warning: Not really an answer to the question, but never the less an alternative...]
I, for one, have given my office chair away!
Or rather... I have replaced it with an exercise ball.
I sit on the ball about half of the day and stand the rest of the day. The advantage of the ball as a chair compared with an ordinary office chair, is basically that you have to use the all the smaller muscles in the back to keep the balance... which is very good for your back and in my case help eliminate the pain I used to have in my lower back... I even bring the ball for nearly all the meetings we have - well, except for those with customers and the top management :-)
EDIT: Another good thing about an exercise ball is that you can use it as... an exercise ball. From time to time when my back hurts whether I standing or sitting on the ball, I use the ball to bend my back "backwards". I simply lie with my back on the ball and hands and feel in the ground for 4-5 minutes... then I'm usually good again...
As for the original question... I don't want to stand up all day or sit down all day. The 50-50 mix is perfect for me.