Did iMessage beta on Lion finally stop working recently?
Yes, Apple did disable Messages beta for Lion users.  French blogger from Consomac found the following in the French strings of the beta:


source http://consomac.fr/news-1339-messages-pas-sur-lion.html

Although this is not the exact error that you are receiving, inevitably it will ask you to move to Mountain Lion now that it has been released.  I encourage the upgrade, and if this problem recurs after you upgrade, try moving ~/Library/Preferences/com.apple.imservice.iMessage.plist to the desktop and restarting your computer.  You can If this does not work, move it back and edit your question accordingly.