You will notice in Defcom's Picture that AIM, @mac and MobileMe are separated by line for Jabber and Google (And as it is an iChat 6 pic from Yahoo as well).
Technically @mac.com and MobileMe names are also Valid AIM Screen Names. (The MobileMe name has to be still currently paid for and active).
In iChat 5 the pic /Screen will vary as you will have the Get an iChat Account button, nor will it have Yahoo.
(if you highlight the AIM option it should take you to the AIM registration page - except they moved it. The MobileMe and @mac.com takes you to the @mac.com registration page)
After you have entered the details and clicked the Done Button then the Server Settings tab will show the AIM server name and ports as it is a Valid AIM Screen Name.
The MobileMe name cannot be one linked or created with iCloud for an iOS5 device (they only work in iChat 6)
The password of @mac.com or MobileMe names have to be 16 characters or less (AIM server limit)
As I mentioned earlier a MoblieMe account needs to be currently active.
This will continue for some until June when MobileMe will end completely (Presumably people will have moved to iCloud)
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