If you have extras-devel configured - install mplayer, and try in a darkish place:
open the camera, do not dismiss the camera, but at least ask it to focus by pressing the shutter halfway. Dismiss camera.
Open shell

mplayer tv:// -tv device=/dev/video0

It's not bad - but you will notice the screen is very dark - and there is no noise.
What happens if you simply amplify the picture?

mplayer tv:// -tv device=/dev/video0:width=320:height=240 -vf eq=77:100,eq=10:100,eq=50:100,scale=160:120,denois e3d=8:6:22,eq=0:99

This is surprisingly good for a quick hack, and generates viewable - though not good - images - when the camera preview window normally shows nothing but black.

your n900 may vary - to increase or decrease the brightness - use the 3 and 4 buttons (you will of course need to use shift).

Obvious things that could improve this.

There is considerable fixed noise in the image. If this was removed first, it would greatly improve the picture quality.

If the camera could be slowed down in frame rate, rather than always running at 30fps or so - it may also greatly improve.

the above mplayer comand takes a 320*240 video stream, and uses three 'eq' filters to simply boost the level of the picture.
Then the denoise3d filter is used to remove a lot of noise, and the final eq filter boosts the levels to make it more visible.

Well, that's not a "Secret Night Vision Mode", so your topic is misleading
It's alternative video streaming from the camera with visual filters.

But thanks for pointing this out, could come handy

So, now we know what Q (oh, sorry R) is going to give 007 in the next movie...

edit the topic, please. now it is totally misinformative

how does -vf pp=al:f compare to that?