Hi,
Try this.
Shut down the Mac. (If it has an Airport Card turn this off first)
Unplug the Ethernet Cable if using that method. (leave the modem connected to the Internet) (remove whichever end is easier to reach.)
Restart the Computer (With no Internet Connection)
Restart iChat (it will remark/protest it has no Internet connection)
Turn on Airport Card or Plug in Ethernet Cable.
Now Log In to AIM with iChat.
Try a Video chat with one of the Apple Test Accounts
See Table 1
http://www.ralphjohns.co.uk/ContactTesters.html
9:04 PM Thursday; December 4, 2008