If you are logged in to gtalk on your N900 and someone who is on a computer (I have tested with both mac and windows) logged into gchat initiates a video call, the N900 will be able to answer and a toggle button to start the front camera will be available. I have used it testing over wifi and live over 3G (this past weekend actually) and it works well.
The picture quality is much better in brighter natural light than under fluorescents or indoor.
PR1.2 should bring the option to initiate a video call.