We managed to do what you want for a college music video project. Not entirely sure how we made it but it involved a piece of wood on the floor with some cogs/wheels attatched so they could spin. We then attached thin scrap metal tubes/pipes to the cog, they had a hole & pin system so you could add more pipes to change the height & then a similar thing to what's on this "sphere arm" of Edwin's to attach the camera.
The actor stands on the wood & the camera operator walks round with the camera attached to the pipes making a perfect circle. You then speed it up in editing.
Not very clear I know, but I hope it helps.
You can see the results at: youtube.com/watch?v=ScxhGEN7Wto
It's @ the end of the video. I plan to upload it to vimeo @ some point but it's on the college computers & I'm on holiday :)

The arm is attached to a tripod.
I want the arm to be expendable so i can reach every spot.
I am still working on it.