Apple offered a beta version of Messages for Mac for Mac OS X Lion users, so they could get a taste of what iMessage would be like on the Mac before OS X Mountain Lion came out. OS X Mountain Lion came out already and has a non-beta version of the Messages application bundled into it.
Apple has sent out e-mails to Messages beta users today explaining that Messages beta for Mac OS X Lion will expire on December 14th. This gives users about another month to use the application before they will have to upgrade to OS X Mountain Lion to continue using iMessage on the Mac:
Originally Posted by Apple
While Mac OS X Lion users won’t be able to send iMessages from their Mac OS X Lion machine anymore after December 14th, they will still be able to resume iMessage activity on their iOS devices and other OS X Mountain Lion machines where applicable.
Oh well… it was fun while it lasted.