Apple introduced the beta of the “Messages” app, and people found out through plist searching, that it will likely stop working after Mountain Lion is released.
Accidentally, I found a simple way to go back to iChat if Messages should really stop working and you don’t want to upgrade to Mountain Lion yet.
Just boot into your recovery partition (cmd-R right after hitting the power button of you Mac), and choose Reinstall Lion, this will not result in any data loss whatsoever, but will replace your main system (again, NOT the apps/data) and you will have iChat back as well as a fresh install of Lion with all your data.
But be sure to have a working backup at hand in case something goes wrong. As always, I am in no way eligible for damages, data losses or losses in general you may have due to this process on your computer.