i had an attny buddy that wanted to use an exercise ball instead of an office chair. ultimately he got made fun of by so many people that he ended up not doing it. i can see startup people using it, but very few will do so in a more professional office setting.
however, i do like the idea of alternative desk arrangements though. i've seen several people with a desk that can be lifted up so that you can stand and work when you want. then there's the treadmill desks which are pretty cool.