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.

sweet works with a magnetic money clip. If you "just" touch screen goes back to sleep. If you move magnet screen stays on. Also does it on N97,but you have to be much closer to the switch.

Magnets induce voltages when moved, and otherwise only affect active parts of the circuit (i.e. which have a current flowing through).
Can you check if the wake-up happens only when jiggling the magnet around or also when standing still?

I opened up my N95 quit often and the sliding screen had a magnet inside. when it moved over the pcb, the pcb would know the screen was open. probably something similar is going on in the N900.

I think Nokia is using a magnetic switch on the back of the screen and on the inside of the case, rather than other mechanical switches (i.e. plungers, etc.) as the magnetic variety will not get filled with dust, etc. I had noticed the same behavior but had not yet gotten the time to write it up. A magnet also holds the kickstand in place... On the N800 a magnetic switch would tell the unit when the back-plate was on or off. It is a very clever way to do it. I would suspect that the camera may work in a similar fashion (although I have not tested that yet.

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.

I have submitted Bug 6689 as an enhancement request.

I noticed this when using a sideways case that had a magnet in it. My battery life totally sucked.

The magnet is making the phone think the keyboard is open. If you turn on the onscreen keyboard and close the slider - place a magnet at the back of the device then do a dmesg from the shell you will see the message:

slide (GPIO 71) is now open

So the phone thinks the keyboard is open when it's not.

Sucks for me since I liked the case with the magnet.

You realize it's impossible for software to distinguish parasitic magnetic fields from the useful ones, right? Don't expect your bug to be fixed too soon

Confirm. I have this issue on N900 and n85....