I just noticed something about the N900:

If you place a magnet near the center of the N900 screen while it is in sleep mode, it temporarily wakes up. If you remove the magnet, it goes back asleep.

I noticed this because I have a leather phone/PDA holder with a magnetic clip, which wakes the N900 up whenever I insert or remove it from the case. It is impossible, with even the utmost care, to insert the N900 without it waking up. It then is easy to press the screen or top buttons and launch an undesired program.

So avoid cases with magnetic clips!!!

I wonder if there is a reed switch inside the N900 which is closed (shorted) by the proximity of a magnet when then keyboard tray is open, and if that explains the waking up behavior.

I also wonder if this can be worked around in software. If sliding the keyboard tray open can be prevented from waking up the device, then it might be enough to prevent magnetic fields from waking it up as well.