When the keyboard tray is closed, I notice that the magnet wakes up the N900 temporarily, but it goes back to sleep when the magnet is removed.

When the N900 is asleep, but the keyboard tray is open, the magnet wakes up the device and it stays awake even after the magnet is removed.

It seems so sensitive to magnetic fields, that maybe it uses Hall Effect sensors instead of reed switches.

(70% right, 60% down) as viewed in landscape mode is the location where it is the most sensitive. If Device Lock is enabled, that would be the bottom right of the <- soft key.