As regardss to the iChat and iChat Sharing categorisation you have used.
In iChat 6 (lion) and Messages (Mountain Lion) the apps login to AIM and to Me.com
The Login to Me.com allows AIM to verify the password to the iCloud issued names (@me.com and @iCloud.com)
This "Dual" login also prevents the newer iCloud issued name from using this site
(it has extra Privacy settings that iChat can't access for your AIM and Aim valid Accounts)
Ignoring the @mac.com and MobileMe entries in the Add Account Screen in iChat and using the AIM one does not get around this issue.
AIM and @mac.com names still work in older versions of iChat.
The short answer is that iCloud issued Apple IDs do not work in iChat 5 or Earlier.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
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