iChat Icon   I can't video chat in full-screen mode

Some video cards do not have enough video memory to support full-screen chats. To have a full-screen video chat, you may need a newer computer or a newer video card.

If you are using two displays, disconnecting one of them may allow you to video chat in full-screen mode.

You can also try decreasing the resolution and number of colors in the Displays pane of System Preferences.