Trying to boot from MMC with a N810 and the latest OS2008 2.2007.50-2 I came across a few issues like mmcblk0 and mmcblk1 being swapped after booting (like vormund earlier). I finally got it right by swapping the devices in the initfs and also managed to prevent the root file system from being mounted over USB. A few pointers here.

For the MMC swapping problem, It may be a good idea to add specific initfs patches in the initfs flasher, but I do not know if this applies only to the N810, to OS2008 2.2007.50-2 or both...