Having recently upgraded my machine to Mountain Lion, I realized that there is no iChat app anymore and that everything ended up into the Messages app. I've been using a
@mac.com account specifically to access iChat, so what happens to this account now? Has it become useless?
Nothing changes for accessing AIM chat using the new Messages app (that integrates iChat features with iMessage features).
Just go to Messages -> Preferences -> Accounts to be sure your @mac.com account is entered with a password and enabled for use.