Apple is notifying Lion users that the beta version of the Messages app will expire on December 14th. The app debuted as a free beta download for Lion users and was incorporated into OS X 10.8 Mountain Lion when the latest version of OS X was released earlier this year.
An email sent to Lion owners that was published by Cult of Mac says that customers must upgrade to Mountain Lion if they wish to continue to use the Messages app. The Mountain Lion upgrade costs $20 and is available from the Mac App Store. With Messages, Reminders and Notification Center, OS X Mountain Lion is a worthwhile upgrade for customers whose Macs are compatible.
You can view a copy of Apple's email below.