Mildly put, Sony's HMZ-T1 Personal 3D Viewer that's capable of projecting a virtual 150-inch screen in a pair of goggles is the stuff of every nerd's wildest dreams. In reality, that's all it is: a display. One modder decided he wanted virtual reality, so he gave it some head-tracking abilities.
The "mod" is rather simple one. YouTube user "iPhwne" simply attached a TrackIR 5 to the Personal 3D Viewer, calibrated the motion sensors to his PC and then piped the video from his system through.
That's about as dirty as the "modding" gets.
As you can see in the YouTube video below, it works pretty well in World of Warcraft and Skyrim. Not sure how comfortable using head-tracking will be over lengthy hours and what kind of damage it could do to your neck, but that's something you really can't worry about if you want to get your virtual reality freak on.