Has anyone noticed that? It's above a tiny, tiny chip, but otherwise serves no obvious purpose.

Any ideas?

I think it is to detect if the back cover is removed and unmounts the internal memory card...

Yup. Have your N800 turned on, remove backplate, place it back. You will get a message about memory card available.

Nice piece of design. Makes me think it might be possible to manufacture a 770-style case with a similar magnet. A software patch would then make the same sleep behavior available from such a case.

The external SD slot cover does the same thing... I opened it when my N800 had the screen off/dimmed, and noticed the screen suddenly go to full brightness. At first I thought my finger had just brushed the screen to wake it up, but indeed it notices when the slot cover opens (and then unmounts the card). Neat!

If you intend to use the magnet detector in the N800 battery compartment you'd be unmounting your internal memory card (and consequently disabling your virtual memory) every time you put the case on (or took it off?)

BTW - the 770 has this magnet detector as well under the battery cover.

Hence the "software patch" I suggested, which would change the behavior from "unmount card" to "put unit to sleep".

Oh right - I hadn't figured you'd disable that feature as it might be considered important, but I guess you'd be accepting the "risk" by disabling the feature! I guess it could be made to work, but might need some major reworking at the rear of the N800... or an extra strong magnet to penetrate through the plastic case backing (remove the existing magnet etc.).

This is a great idea! I never pull out my memory card, so I could just unmount it from XTerm if I needed to, or just turn it off and remove it.

Seriously, having to turn the unit off to service the internal card is a small price to pay for the extra fun that can be had with this sensor.