Apple may have released a public beta version of Messages for Lion yesterday, but from what Consomac has discovered, it may only be a temporary situation. Digging into the text strings inside the app's resources found one particular string which said:
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.
This suggests that once the Messages beta has concluded, the app will cease to work and users will have to purchase Mountain Lion to continue using the Messages app.