Most digital cameras (including the one on the N900) can still pick up infrared to some degree, despite the IR filter.

For an example, start the camera app, point a TV remote control into the lens and press a few buttons, you'll be able to see the IR LED in the remote light up on the N900's screen.

If you picked up a powerful enough IR torch you might be able to take night photos without any visible light. Maybe. But like Mandibela said, you're probably not gonna get much of a picture without busting open your N900 and removing the IR filter.