Hmm, I was wondering for a way to boot Maemo without using MTD5 at all, freeing MTD5 for mtdswap.

Anyways, as long as you have a complete filesystem on sdcard (and fstab regeneration disabled -- see ext2/3 on /media/mmc1 thread) and a proper fstab made, I don't see why Maemo wouldn't boot from sdcard.