Vuzix is now shipping their Wrap 1200 3Ds, a pair of $500 glasses (a headtracking model called the 1200VR is coming later this month) that displays a 75-inch virtual screen in front of your face and supports 3D content.
You have separate focus settings for each each eye and these are as light and small as a standard pair of sunglasses.
The Vuzix 1200s also allow you to wear your own prescription lenses under the device.
I’ve used earlier Vuzix video glasses on flights before and, barring the dork-feel of wearing a pair of video glasses, the experience is fairly interesting and impressive. Now, however, with HD, 3D and a huge screen these things could, feasibly, replace a standard monitor in some situations.