I am not sure what you mean.
I have been to my Address Book on my G4 and done similar to what I posted to Defcom to make the point.
I changed my Last Name using the Edit Button to
G4 So now the G4 computer reads that the Bonjour Buddy List reads Ralph G4 and the G4 displays on the MacBook Pro as Ralph G4
I have not changed the Address Book entry for the Me card on the MacBook Pro so it read as Ralph Johns.
On the G4 I have a Address card for the MacBook Pro and one for the G4 that essentially are both Me. (The Me Card is attributed as described).
I use just the Screen Name entry on the MacBook Pro in iChat and have not bothered with a second Address Book card for myself there.
There is an entry point for your Profile (AIM Only) that is in the iChat menu.
If you mean you are someone else's Admin account on a Mac and have a AIM or Jabber Name in their iChat that you use, the Bonjour will still list them as the Scree Names there.
You would have to have your own Address Book entry and change their Me Card to your entry. Remember to change it back as it will be the entry on the Header info on their Emails otherwise.
8:41 PM Tuesday; June 3, 2008