I saw on Ray Stephenson’s blog a video of an omni-directional treadmill, commercialized by Virtual Space Devices, Inc.

Virtual Space Devices, Inc. has developed omni-directional treadmill (ODT) technology over the last 10 years, working primarily with the US Army. Combining the infinite plane of the ODT with an immersive virtual reality system permits an immersant to walk or run freely through the digital landscape. Navigation is transparent and natural. Immersion is total.

The ODT surface is flat and continuous. It is not a bowl or made of discrete plates. The infinite surface is motion-slaved to the user’s motion with tight control and no inertial residue. Belts that comprise the surface are substantially similar to those used in conventional treadmills.
First work on omni-directional treadmill (ODT) technology began with a $400K US Army STRICOM contract in 1996. (…)
Second-generation technology is larger, faster, and quieter. We are building multiple units

More information in this article [pdf].

This may be an alternative to the Virtusphere. Wondering what the price range is?