Be careful here - I am lead to believe that the N900 has a magnetic sensor that detects when the back plate has been removed and automatically un-mounts the SD card. There is also evidence that this magnetic sensor can get triggered by external magnets like the ones some people use in pouches. There is a bug logged for this but I cant for the moment find it.

It would be somewhat disastrous if the SD card was occasionally unmounted while being used as swap would it not??