Yupe, it only works if the video call is initiated from a computer, or in your case, n810. It however does not allow video call if the call is made from n900.
Anyways, it had been discussed couple of times a while back, for example:
http://talk.maemo.org/showthread.php...ighlight=gtalk
http://talk.maemo.org/showthread.php...ighlight=gtalk