I've actually seen an omnidirectional treadmill. I was being given a tour of a cool company on a weekend by one of the employees that is a family friend. I didn't get to see it in action, but it was a giant square with a treadmill of teadmills (going different directions, to allow for omnidirectional movement.) It also had a frame for mounting cameras. When operational, the cameras would be able to detect where someone is on the treadmill, and move them back to the center. So if you wanted, you literally could run as fast as you can, and still wouldn't be able to get off.