In iChat 4 and iChat 5 there is a button that says "Get an iChat Account" and did go to different places depending if you selected AIM or @mac.com/MobileMe.
AIM took you to the AIM registration page (Until they moved it)
@Mac.com and MobileMe took you here to the @mac.com registration page
These names still work.
The issue is if you either set up or change the password so it is more than 16 characters.
16 Characters is the max size of a Password that AIM servers can handle (it rejects the whole thing rather than truncates it)
You will need a separate email address to those you have already used in setting up your previous Apple ID at @mac.com
(It sort of Hard Links the external email to the @mac.com name and you then cannot use it again to set up a new Apple ID)
I am not sure why Apple have made this change to iChat 5 particularly as the web Site Registration is still there.
The Button has disappeared in iChat 6 and the Messages Beta
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.3)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously