Can I uninstall the Messages Beta? I'm thinking about trying the Messages Beta on OS X Lion. I'm wondering if I will be able to uninstall it if I don't want to use it anymore. If so, will I be able to use iChat again?
Yes. 

In the Messages menu, there's an option to uninstall the beta. Apple just messes with the App bundle to add support for the iMessage protocol. The underlying app is pretty much the same.

See this AskDifferent Answer and this post which it references for more information.