To be clear.
MobileMe issued @me.com name no longer work
@MAC.com names will
They are regarded by the AIM Server as two separate Valid Screen Names (providing the password is kept to 16 characters or less)
If you converted an @me.com name to iCloud this will only work in iChat 6 and Above (Messages)
This is because the post Lion apps do a login to me.com as well as AIM which allows AIM to "See" the password.
In iChat 6 and Messages the Add Account screen/pane only has the AIM drop down (And not the variants iChat 5 and earlier had)
Any Apple ID that is going to be used in Messages as the iMessages account is set up on launching Messages for the First time.
It is added to the list as if it were an "Account" but you cannot set up more "Accounts" of this type in the Add Accounts pane.
You can only look in the Account Details button and then Sign out of the current Apple ID before listing a replacement ID
To be really clear.
Apple issued IDs ending in @Me.com and @Mac.com can do several things
As an MobileMe that followed an .Mac Account, you may have linked the email services and used the two names Interchangeably in iTunes, Apple Discussions, App store and some other places.
However as far as AIM are concerned they are Separate Screen Names (They just happen to have the same password).
The iMessages Account is an account only as far as the Preferences Listing is concerned
It or another cannot be added as a new "Account" (only Screen Names and Jabber IDs can have that done)
It also does not have a Buddy List (It accesses Contacts via the Messages (main) window)
The verification process has changed that allows AIM to "See" the password on some Apple IDs and those registered with iCloud can only be used in iChat 6 or later.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.2)
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