I can definitely confirm it's a magnet which senses whether the backcover is on or off. i used the magnet on a generic case I bought for my N900. I was able to enable the SD card with the cover off by holding a weak magnet over the camera area.

So it looks like that magnet on the backcover is doing double duty. Keeping the kickstand snuggly held closed, and to tell the phone whether the backcover is on or off.

Additional note. There is a also a magnet and switch which are used to determine if the slide keyboard is open or not. It's located on the right side of the N900 well away from the backcover magnetic switch/sensor near camera. The use of these two magnet probably explain why there isn't a magnetic compass in the N900.