Hi,
As Defcom says the Mac User Logged in on the Mac should be the same person as the Head and Shoulders card (My Card) in the Address Book.
Options are to Change the Address Book entry if you are the only one using the Mac.
Create your Own Mac User account on that Mac.
Change the Display to your Handle and forget about it.
On the Last.
If your Colleague takes copy of the Address Book to another computer the Cards will have the info about your Buddies Screen Names (At least that have Real Names connected to iChat) even if he does not have access to your Buddy List.
If you move computers (or even User's Account) then wipe the Address Book and the Keychain of info relevant to you.
8:30 PM Saturday; January 30, 2010
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"