Go to the Messages Menu.
Click on the line that says Uninstall
This will delete Messages beta and reinstall iChat 6
Of course Messages will be released in Mountain Lion.
How close it will be to the Beta is not known (Or if it is, people cannot say due to NDAs (Non Disclosure Agreements)
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously