[rudasn]

My bet is on brain waves/some sort of EEG. I don't know much about the subject but at the moment it's impossible for a device like Glass to function as an EEG but I think we are not too far away (maybe 10-15 years) from figuring out a sexy solution.

http://www.emotiv.com/

[r0s]

Once you dig into the technology a bit it's somewhat disappointing. The best layman summary I can make:

EEG or other electrical signals are scrambled in the skull, bouncing around inside, making discovery of origin impossible from the exterior. The best we can do is binary on or off, which is how the emotiv device works, along with detection of facial/other head muscle movements.

MRI can detect origin of signal, but the machines are huge, like the size of a car; and power hungry, and crazy expensive, and the resolution is something like one minute per signal.

Sorry to be a bummer ;) I want to see brain-computer interface as much as anyone.