http://www.newscientist.com/article/dn27555-your-brains-unique-response-to-words-can-reveal-your-identity.html#.VV6MhHgQ3EY
Though... having unique thoughts for every login could get pretty confusing.
There are so many problems with bio-metric passwords. This will be another one of these "coming-soon" technologies that our grand-kids will still probably hear about. I don't see this one coming out of the theoretical realm anytime soon, if ever.