To say this another way.
Any @mac.com name works as an iChat screen name "forever" with the AIM service.
This means any Trial account that was ever made, any Lapsed account held in the past, any Linked with a MobileMe Email account (Separate issue) will work as a Valid AIM screen Name in iChat.
Notes:-
At the .Mac service to MobileMe service change people were offered the chance to link an @mac.com Name with an @Me.com account for Mail purposes.
These people have then got two valid AIM Screen Names.
MobileMe Names (Ending @me.com) stop working once the Tiral or an account itself is cancelled or lapses.

8:40 PM Monday; June 13, 2011
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb( 10.6.7)
Mac OS X (10.6.7),
"Limit the Logs to the Bits above Binary Images." No, Seriously