When I formated my microSDHC as ext3 I started to get those annoying messages, too. i only got rid of them when I've changed the "mmc-mount" and "osso-mmc-mount.sh" scripts.

It seems that "osso-mmc-mount.sh" verifies for each file system if it is in a supported list (which only have a few allowed IDs). When I included id "83" there, and changed mmc-mount it stopped complaining. The n900 also started to mount the microSDHC automatically.

It might be the same script in your case. It might be going through all available partitions and feeding them to this script or similar...