This Friday, Apple notified customers that the company will be discontinuing the Messages Beta program for Lion on December 14th, 2012:
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.
The email also mentions that in order to continue to use Messages,users must upgrade to OS X Mountain Lion. OS X Mountain Lion is available for $19.99 on the Mac App Store [Direct Link].
Messages app is Apple’s iMessage client for the Mac. The client also replaces iChat in OS X Mountain Lion.