I would like to add, that this sensor detects white/black even in absolute darkness (probably uses something out of visible light wavelength - IR?), and of course mattkawa is right, that no magnet is involved in camera-slide state detection.

I suspect that Your sensor is damaged, and only some kernel hack (enabling sysfs entry?) could allow to "cheat" system. Of course, I can be wrong.

/Estel

// Edit

woody is probably right - magnet doesn't matter in slide open/slide closed detection, yet, it may shut off completely whole check. I have magnet glued for good on appropriate sensor on N900 side, to have possibility of removing cover without disabling microSD (very important, if You use swap on microSD, hah!), so I could just not remember about that.