What's the probability of doomsday on Dec. 21, 2012?

Probably no greater than the Y2K doomsday. The Mayans didn't predict doomsday on that date anyway. It's just the date of the end of their calendar & supposedly the beginning of a new age. It's also said to be the day the aliens land by UFO enthusiasts. But most of them feel they've already landed anyway, so that's a moot point.