Here's a complete explanation by my friend (and new Slate blogger) Phil Plait of why the world isn't ending today, despite all the nonsense about the Mayan Apocalypse.
I don't have any interest in predictions about the present made a thousand years ago by members of a dead civilization -- especially since they didn't actually make any such predictions. Modern-day fear-mongerers have misinterpreted the end of the Mayan calendar as the end of the world. I'd wonder if they've ever bought one of those 16-month calendars and thought the Earth would blow up at the end because they ran out of days, or if they just went out and bought a new calendar. Perhaps the Mayans simply got tired of carving more dates this far into the future.