I was helping a friend set up a MacBook. When we entered his username xxxx.yyyy@mac.com and password into iChat, the credentials were rejected by the server ("Incorrect password"). Though I was sure the password was correct, I reset it via the web, but to no avail.

Finally, having run out of other things to try, I entered the account as an AIM account, with "@mac.com" appended to the username. It worked straight away.