Reminds me of the story about the man (a drunk) and the street lamp:

A man came across a drunk looking under a street lamp one night. The drunk said he was looking for his keys which he had dropped. After helping him search unsuccessfully under the lamp he asked the drunk exactly where he had lost his keys. The drunk replied, 'Somewhere over there in the dark, but I can't see a thing over there so I'm looking under the light instead.'

If you can't find your N900 in the dark, you'll probably not find any other thing in the dark also - Nothing to do with the N900 or apps. Help yourself and do something with the lighting...maybe an emergency light or something. Don't worry that you can't find the N900 when it rings. You can still follow the sound and/or whatever light on the display in that total darkness will be visible (even upside down).