I ran into the same problem. My mmcblk0p2 partition started to fall apart with a lot of badblocks and read errors.
I reflashed successfully with the RX-51_2009SE_20.2010.36-2_PR_COMBINED_MR0_ARM.bin image.
then I tried to reflash the emmc and it failed.
After a lot of reflashing of the os image i finally got into the situation where i got the os installed and the mmcblk0p2 patition unmounted due to filesystem failure.
Then i put the n900 in r&d mode.
Ran
badblocks -w /dev/mmcblk0p2
several times.
I seems like the pattern writing by the badblocks application did the same job as a flash formatter and i got into a situation where there where o badblock left.
Then I ran
mkfs.ext3 /dev/mmcblk0p2
and
fsck.ext3 /dev/mmcblk0p2

Finally a last reflash of the os image

And to my surprise the phone was ok again!