Thanks to a text string Consomac found in the beta version of Messages, it looks like Lion users will get cut off from the app once Mountain Lion is released. The text string reads:
Thank you for participating in the Messages Beta program. With the inclusion of Messages in OS X Mountain Lion, the Messages Beta program has ended. To continue using Messages, please visit the Mac App Store and purchase OS X Mountain Lion.
The public beta of Messages for the Mac lets users instant message through AIM, Jabber, Yahoo! Messenger and Google Talk, but also adds SMS messaging that sidesteps cell carrier texting plans. With Messages already available as part of iOS 5, conversations are synchronized across your Mac, iPhone, iPod touch and iPad.
Apple introduced Messages for the Mac as a new feature in OS X Mountain Lion, which was also unveiled on Thursday. The new version of the Mac operating system adds more features from iOS for the iPad such as Reminders and Notification Center.
The public beta of Messages for the Mac is available as a free download at the Apple website.