An update...

partitioned and formatted the card over USB from a linux PC, then on the n800 copied the OS over to the card. Results are the same, unable to boot from the mmc.

Also tried partitioning the card with a ~4GB fat32 partition as mmcblk0p1 and the other ~12GB ext2 partition as mmcblk0p2. This also failed to boot.

Thanks,
Andy