I recently ran across this (I don't know where, maybe at WMMNA): Powered Shoes, a project carried out by Hiroo Iwata. It's basically a "wearable locomotion interface that enables omni-directional walking while maintaining the user's position"

A locomotion interface using roller skates actuated by two motors with flexible shafts. The device enables users to walk in arbitrary directions in virtual environments while maintaining their positions.

Enhanced Life It has often been suggested that the best locomotion mechanism for virtual worlds would be walking, and it is well known that the sense of distance or orientation while walking is much better than while riding in a vehicle. However, the proprioceptive feedback of walking is not provided in most virtual environments. Powered Shoes is a revolutionary advance for entertainment and simulation applications, because it provides this proprioceptive feedback.

Why do I blog this? It reminds me of something discussed with julian about a walking-based interface. Lots of interesting mixed reality application could be used using this sort of device: not in the proper "mixed" system that actually exists (including virtual world features in the real world through glasses) but rather by allowing tangible interactions to control stuff that would happen in the virtual worlds.