In brief, you can not initiate a video chat yet, only voice, however you could activate video once your interlocutor start the video call. You can enable video from the N900 and can have your very own video chat. (two-way video and voice)

Once you click on "Camera" you can see the video of your interlocutor in the middle of the screen and your own video on up-right corner !

It's just awesome !

Via : talk.maemo.org