Publisher THQ has filed a trademark application with the US Patent & Trademark Office for the number "1666." Given the classification, the filing is likely to be used as the title of an as-of-yet unannounced game, though THQ's not responding to requests for comment at the moment.
Gamespot postulates that the title is a refrence to the Great Fire of London, which destroyed a huge swath of the British capital in September of 1666. It's worth noting that only six people are known to have died during the fire, though, and that's not exactly a video game caliber bodycount.
Joystiq suggests that the might also be referring to the Four Day's Battle, one of the longest naval engagements in history, which occurred in the same year.
We'd like to throw out hat into the theory ring too, but since all the good ones have already been taken, we'll have to settle for something crackpot instead. So you heard it here first: the title is actually an area code — 1-666 — and the game is about a phone that's been possessed by Satan.