Here's something interesting to check out at the upcoming Consumer Electronics Show. Murata Girl will be in attendance. This unicycle-riding robot comes courtesy of developers at Murata in Japan who based it on an extension of an earlier bicycle-riding Murata Boy robot.
The trick, according to Murata Girl's Web page, was in devising controls for four different axes that didn't accidentally twist the robot as it moves. Apparently the design is a success. You can watch videos on the site of Murata Girl riding a conventional balance beam and even an S-shaped beam. There is also a short commentary there from developers about how they went about designing Murata Girl, though they provide few technical details.