MacBook brightness set to zero suddenly resets itself to one. Any ideas why? Every night I start some music/audiobook/whatever in iTunes and then reduce the brightness of the screen to zero to listen to the audio without seeing a bright screen. For the past few weeks (I think), my MacBook (2007) automatically increases the brightness to at least level one after five or ten minutes requiring me to press the lower brightness button again to switch the screen back to dark.

Any idea why it does that?
The 2007 MacBook (without "Pro") doesn't even have an ambient light sensor, so that one can be ruled out. My guess would be that some software, maybe even the system itself, turns the background light back on to let you know that the machine is still running.
If you're on Snow Leopard or Lion, you could use the Screen Saver prefs to assign a "Hot Corner" to "Display Sleep" which turns off the display without putting the computer to sleep.