This would be nice to install on a fresh os flash... where you might flag the internal 256megs to be a read only partition and union mount a partition on sd.

When you screw it up just create a new partition on mmc and union it back to original fs.

The eee does this with their xandros os install... its readonly partition and the other partition is user files/progs.

Additionally this would have added benefit that you could back up the mmc image and its basically just backing up whats 'different from base install'.