Hmmmm. Strange one.
Have you had a close look at the contacts in the memory card slot? Maybe one of these could be slightly out of shape, also try giving them a gentle clean with a pencil eraser, and maybe do the same to the contacts on the back of the card. See how that fares up.

And try starting x-terminal and typing: dmesg | grep mmc0 and see what it reports.

I take it the magnet on the back cover of your N900 is OK. Other than the above, you could have a hardware problem with the 900.

Sounds a bit drastic, but not sure if reflashing the 900 will fix the issue.

P.S. Sorry for leading you off to an 810 thread, should have checked that before posting the link.