How to boot from SD?
Well, preparing an SD is trivial; I can document it if you want, but in essence..
The chip has a limited filesystem boot option right in it, which makes the panda unbrickable; it knows how to read a very specific FAT setup which in turn houses the kernel, and the args you pass (using a bootscript ideally, so you don't have to type) tell the kernel what to do from there; the ideal setup is two partitions .. FAT to boot, and then most of the SD as ext2 or ext3 (say), with the OS image on it. Alternatively, you can do two SDs, one for FAT for boot, and the other for the image (say); this is how we do development.
So you want to boot another OS.. prepare an SD (partition, copy kernel over, unpack the OS onto the second partition) and you're done, good to go, can boot that one instead of the one on NAND.
Naturally, spare permitting, you can partition your NAND up and stick the other OS on there, and then boto from that, based on an SD-menu say. Or if someone wanted to make a bootmenu, could be possible to switch OSes and boot it all from NAND.
So yeah, multi-OSes is piece of cake.
jeff