A little more detail on the name would help and also the iChat version number.
Apple IDs issued under iCloud (@Me.com or @icloud ending names) will not work in iChat 5 or lower
This is because the checks iChat 6 and Messages make require the app to be logged in to Apple as well as AIM to then be allowed to "read" the password.
In All cases (Includes @mac.com names) the password on Apple IDs that work as AIM Screen Names have to obey the AIM limit of 16 characters.
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