Hi All,
There have been several "Apple" IDs over the years (Names issued by Apple)
iCloud issued @me.com names are different to MobileMe issued @me.com name and @mac.com names
All ARE Valid AIM Screen Names.
However iChat 6 and Messages Beta both login to me.com as well as AIM.
This fact allows AIM to verify the password when the name is used in iChat 6 and Messages beta.
iChat 2 through 5 do not do this dual Login so the Password cannot be verified by AIM and so the iCloud issued names do not work in iChat 5 or Earlier.
If you have an @mac.com or MobileMe issued @me.com then you can go here (AIM Settings) and create Display Names, adjust Off Line IMs setting and other things.
You cannot do this with the new iCloud issued name due to the lack of the dual login.
If you are using such a name (iCloud) in iChat 6 (or Messages beta) you still need to keep the password to 16 characters or Less for the AIM servers limit.
Use the AIM settings link and follow the "Get Username" Name link top right or use
The @mac.com registration page to create new names for iChat 5 or earlier.

10:09 PM Wednesday; April 11, 2012
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