What will happen to iChat in Lion once Messages beta expired?
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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
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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.
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For the complete information I will point you to Dr. Drang post that answers this question, and more, about iChat/Messages.