I found this article very interesting. I’m not aware of any MMC/SD-boot related bug fixes in the romcode and almost all Rev B boards have the same romcode as Rev C boards.
I wonder if this really is booting the MLO off of the SD card, since the NAND-based MLO doesn’t have difficulties reading the u-boot.bin on SD cards that don’t follow the same formatting restrictions as the romcode.
Certainly the howto is more restrictive than the romcode actually requires, but the point was to provide information that always worked. I believe at least the part about marking the partition bootable is required.