To follow up on Tom McDonald's post, here is more info on the discovery;

David Stuart, who wrote the report with William Saturno, has informed me that the press has the story wrong. This is not the earliest calendar found. Here is what he wrote me;

"The Xultun tables do NOT represent the earliest Maya calendar(s). The media has run with this meme, but it's not what we said at the press conference. Just want to clarify."

William Saturno has found a very early Maya calendar in wall paintings, dating to the 9th century, charting the motions of the moon and Mars and Venus. The drawings were found at the site of Xultun in Guatemala. The 360 day "tun", the 20 day "final" and the one day "k'in" are represented. Lunar semesters of 177-178 days are calculated. It looks like the lunar "semester" was calculated on the calendar for 13 years. The finding is reported on in the May 11 issue of Science. Another table represents the 18,980 days of the Maya calendar round. It appears they were also notating the movements of Jupiter in relationship to the moon.

Scientific American has the report here;
http://www.***.com/

And the NY Times added this bit of important information to the story;

"The principal scribe, who may have been related to the royal family, also left his mark on the north wall, near the presumed kings picture. Four long numbers there represent dates that stretch over 7,000 years. The scientists said this was the first place that seems to tabulate all these cycles in this way. Another number scratched in the plaster records a date that translates to A.D. 813."
http://www.***.com/

The Boston Globe has a slide show of the find here;
http://www.***.com/

And National Geographic adds that the murals were painted during a time of intense drought at the site and folks may have been looking to the scribes for answers.
http://www.***.com/

And the BBC adds that the dates written on the mural stretch 7000 years into the future, thus disproving the Maya end of the world theme.
http://www.***.com/