Neat idea - small screen, put into binoculars-like mount, connected to N900's TV-OUT, and bundled with powerful pack of IR diodes. N900, of course, with IR modified camera module.

Result:
N900 turn-able into NightVision set, with surprisingly good quality/price ratio. All You need is to replace day-photo module to prepared IR one (require ~20 gram small screwdriver with two tips) - can be done in 3 minutes, when You gain experience with quickly assembling and disassembling device - and, plugging binocular's plug into TV out.

With some smart construction, whole set may be extremely lightweight, ultra-portable (to the point of always having it with Yourself), and non-requiring another, dedicated N900 (compatible with main device Youi're using everyday.
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Crazy idea for my personal variant of custom body replacement - mechanism for camera module quick-swap, without need to disassemble whole body. Just "click", and camera module is removed. Another "click", and 2nd one is back in place

I don't think it would meet widespread usage, but for me, personally, it's another overkill feature.

/Estel