Since installing the beta version of Messages (on day one), I have quite enjoyed its capability to sync with my iOS device messages as well as Google Chat, AIM, and other Jabber accounts. Today though, Messages did not open as expected.
At first I thought it had something to do with the beta version running out. Recently, Apple took down the capability to download Messages, so perhaps disabling the beta version was part of the plan all along. But, that doesn't really make sense without a clear replacement intact.
With Mountain Lion still being tested, I needed to find out what was going on. I found a thread on Apple Support Communities describing my issue. Several suggestions were presented, none of which seemed to be doing the trick. Deleting preference lists or re-installing the app had no affect.
Then, one suggestion, courtesy of jmw1480 did the trick, and it's probably the easiest one to execute.
To restore functionality in Messages beta, quit the app, open Activity Monitor (I use Spotlight to launch apps that are not in my Dock), and search for "imagent" in the processes list.
Click on it and click Quit. Confirm that you'd like to quit the process and you're all set. Open Messages beta and you should be good to go. Keep in mind that Messages is a beta application and things like this are almost certain to occur. With the release of Mountain Lion, expect to see a much more stable, full version of Messages.