Philipp Lenssen has the scoop on a new entry into Google's robots.txt file -- "mbd?". There are some guesses as to what this could be in the comments for his article. One interesting suggestion that would link directly into Google Health was "Medical and Biological Database".
A search on AcronymFinder.com points out that MBD can be an acronym for "Marketing & Business Development" which I think is probably more likely. Does anybody else have an idea what this could mean?