While it's not exactly a Star Trek Holodeck, VirtuSphere is definitely big step in the world of virtual reality: it eliminates bumping into walls!
The VirtuSphere platform consists of a large hollow sphere that sits on top of a base and allows the sphere to rotate 360 degrees. Wearing a wireless, head-mounted display, users can step inside the sphere to fully interact in immersive virtual environments. The VirtuSphere enables 6 degrees of freedom – one can move in any direction; walk, jump, roll, crawl, run over virtually unlimited distances without encountering real-world physical obstacles.
Link - via Cribcandy