Currently Being ModeratedSep 17, 2013 1:17 PM (in response to Niki Tatum)
iChat can use certain Apple IDs as AIM valid Screen Names.
They have to end with @mac.com or be issued since iCloud came about and have @me.com or @icloud.com endings.
However Messages in Mountain Lion can use and Apple ID with the iMessages account.
This can be any form of Apple D that you have.
iMessages needs to have different IDs on different computers if you don't want them to sync the same messages.
iChat effectively needs different Apple IDs or AIM Screen Names if you want to separate them out.
If you registered for iCloud near the start you may have been given an @me.com ID. Later you may have been given an @icloud.com ID.
These can be used as two separate AIM valid Screen Names (You have to keep the password to 16 characters or less (An AIM server limit) )
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously