Apple will be shutting down Messages beta for Lion service on December 14th, 2012.
Email from Apple:
Dear <insert name here>,
The Messages Beta program for Lion will end on Friday, December 14, 2012. We hope you’ve enjoyed the opportunity to preview Messages.
If you’d like to continue using Messages, upgrade to OS X Mountain Lion from the Mac App Store. Messages is one of many great new apps and features built right into OS X Mountain Lion. Learn More.
Thank you for your participation in the Messages Beta program.