Smyth and Yamaha
Smyth Researth has been showing their headphone surround simulation technology (Smyth Virtual Surround or SVS) for a couple of CEDIAs now, and it looks like it is about to come to market.
The system analyzes the physical structure of the user's ears (the configuration of the outer ear and outer part of the ear canal, not the individual's hearing capability itself). With this information, it can simulate the effect of a full 5.1-channel system using earphones. There are other technologies that claim to do the same thing, but I can state from prior show experience that SVS actually works.
According to Smythe, An accessory processing box, together with headphones, in-ear microphones for analyzing ear structure, and a remote, will be marketed by Yamaha early next year, though an ideal realization would be to incorporate the circuits in an AV receiver or pre-pro.