The magnet is there for a quite important reason: The N800 _must_ detect that you may be about to remove the SD card, before you actually do it. This is because it must unmount the card, which means to write any non-written data to the card first to avoid card corruption. This takes a non-zero amount of time.
When you close the cover it re-mounts the card.

This could of course have been done by some other way than a magnet, e.g. by a mechanical cover-closed detector, but that involves moving parts which sooner or later may stop working. A magnet is a quite elegant solution, if only the glue was a bit better..

For the external slot there _is_ a mechanical sensor, connected with the little latch that closes the SD slot.