Messages will replace iChat on OS X Lion when installed but it will expire at some point in the future.
What will happen to my Lion's iChat application when Messages expire? Will I be able to use/reinstall iChat again?
So there really isn’t a Messages.app. It’s still iChat.app but with some magic causing the Finder to show it as Messages. [...] [T]he magic is in the Bundle name entry in the Info.plist file in /Applications/iChat.app/Contents
Why did Apple choose to do it this way? My guess is that Apple wanted Messages to inherit all your old iChat settings so you wouldn’t have to redo everything. Also, because Messages is in beta, Apple wanted you to have an easy way to uninstall Messages and go back to iChat as if you never left. I’m not sure this shallow rebranding was the best way to achieve these goals, but it does seem minimally disruptive.
For the complete information I will point you to Dr. Drang post that answers this question, and more, about iChat/Messages.
In the Messages menu you'll find: Uninstall Messages beta. Choose it and a dialog will ask if you'd like to reinstall iChat. I haven't yet followed through with that but I'm assuming iChat is still stored on the computer somewhere or gets downloaded and installed from Apple's servers.
This is pure speculation on my part, the worst kind of Ask Different answer, but it's based on past experience with the Apple ecosystem and supported by the technical answers already provided:
You can rest assured that Apple won't leave Lion users who have downloaded the Messages beta without iChat capabilities. The Message client will either be reverted to an iChat client or, more likely in my view, a non-beta Message client will be made available for Lion.