Me got this issue as well. I found there is bug exists in bug tracker - see bug 6689. I have idea how to workaround this in software, see comments to a bug.

The N900 has a problem with magnets. The screen goes on when you hold a magnet nearby the screen. This happened also by the back of the N900. If you have a case with magnet closing (my old E90 case from Carrymobile) like i have , see my YouTube film. Has somebody the same problem ???
http://www.youtube.com/watch?v=kaYT-cqCjdw

Oh yeah. You have to test a lot of cases. I finally found one -- a cheap off brand -- with a lightly weaker magnet that seems to work. Most cases have a stronger magnet that will set off the N900.

And you can;t find a case without a magnet, it seems.

Good luck.

When i use my magnetic pouch for the n900 i face the screen inside instead of outside because of the magnetic field. This prevents my n900 from turning on whenever i try to pull it out. I've have tested it many times already. I have an original Blackberry pouch for my n900 which fits perfectly. It's a slide down pouch unlike the one in that video. I recommend it.

I have a case from Tmobile (USA) store and has no magnet. the case is just like ones with magnet but has no magnet. Width is just fine but is a bit longer than n900. But it's a good fit. I used to use it for G1.

i prefer the invisible protection like zag and a leather slip case.

this has been discussed before - the hardware for detecting keyboard slide is most probably based on some kind of a magnetic detector...

And for the back - the camera slider has a similar setup.

will this mess the phone up over time?

Threads merged.