I always thought that the Mayans believed in "cycles" of the earth and that Dec. 21, 2012 was simply the end of whatever cycle we are currently on.
As stated, if they were so awesome at predicting things why aren't they still around to tell us the winning lottery numbers or something useful.
I think you're right about the cycle belief, but I believe their calander continued through the cycles, and just abruptly ends on the said date in 2012.
Also, if memory serves me right, Edgar Cayce predicted a similar date. The chinese I Ching also stops on the same date. The specific date has been recorded throughout history by different people, in different places at different times.
Even Einstein had mentioned that the poles have shifted in the passed. Could this be the date it happens again?