Sony has an HMD on the market right now, but from what I understand, it's not quite what we all expected "virtual reality" to be. HMDs still have a lot of shortcomings, but they're getting closer to that grand vision of total immersion. The significance of Sony's unit is that the resolution isn't the laughable 256x160 or so that a lot of other HMDs have had. It's actually 720p, and stereoscopic. It still doesn't fill the entire field of vision, which is the next obstacle that needs to be addressed. When we see affordable HMDs that have wraparound FOV, I think we'll begin to see games designed specifically to take advantage of them.
I don't think this is far off at all. Combine the mythical "mainstream HMD" with Kinect for head and limb tracking and you have the 1990s dream of virtual reality. When we get there, what kinds of games would most benefit from it? How would achieving that goal affect gaming? How would the control scheme for a first-person game translate, given the fact that you still can't get up and walk around or turn your head all the way around?