Looks like Portal 2 has slipped back some two months from its original release date, but this is something Valve are actually proud of, as they explained to us via email.

Valve today announced that Portal 2 -- the sequel to the ground-breaking title that won over 30 game of the year awards, despite missing its original ship date -- will now be available the week of April 18th, 2011. This two month slip not only marks the shortest delay in Valve's proud tradition of delays, it represents the approaching convergence of Valve Time and Real Time. Though this convergence spells doom for humanity, it will not affect the new Portal 2 release date.

Perhaps the convergence of Valve-Time and real-time will happen in 2012, meaning the Mayans were right.