This is somewhat a hangover from the iChat days.
In Messages > Preferences > Accounts is the GoogleTalk account listed as Gmail item ?
IF you highlight it and go to "Server Settings" is the server talk.google.com ?
Now go to the Window Menu
Select the Buddies item (you probably still have a combined list)
The Buddy List should now appear.
GO to the Preferences > General section and untick the item about gathering Buddy list into one list
You may now have more than one Buddy list
Instead of being called "Buddies" it will take on the name of the Account ....
The exceptions to this are:-
Generic AIM Logins will be Called AIM Buddy List if there is only one
Generic Jabber Logins will lbe listed as "Jabber" Buddy list if there is only one Jabber list
(you can also have Yahoo and Bonjour)
IF in the Preferences > Accounts you add details to the Description Line then the Buddy List gets called this
IF you don't name or Describe the accounts and have more than one (Jabber and /or AIM) then they get named with the Screen Name or ID.
Account and Buddy list "Named"
This now carries over to the text entry field/box in the main Messages Window where the ID or Screen Name you will be chatting from is listed in that Text field.
IF you only have one Jabber Account And Googletalk is a Jabber account it will read "Jabber" unless you have more Jabber accounts or add a Description
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
I too had the the same problem, but i solved it by the followin method.
Step 1 : Enable 2 setp verification from gmail.com (https://www.google.com/settings/security)
Step 2 : Go to Manage in Connected applications and sites tab
Step 3 : Generate application specific password.
Step 4 : Now go to system preferences > Mail, Contacts & Calenders > Google tab (If already) > details> enter your username and then the application specific password
Step 5 : In iChat preferences , you'll now see the google acc with greyed out username and password and a single option of Enable this account.
Step 6 : Now it should work