For those who are afraid to use a public image because they don't know where the image has been...
I'm sure I did several things in a very silly, unreasonable way. That's fine. I was using a fed13 image I made that used the Ubuntu AKI before; spent part of the last couple days getting this working. Follow the simple instructions and you too can have an ebs-backed, pv-grub'd instance built how you want it (just look for the yum --installroot=/mnt/chroot lines, add what you want).
I'm absolutely positive that others have much, much more mature setup tools avail for doing the same thing, but someone might find this useful anyway. And a hat tip to Justin for pointing me to his AMI to peek at, which tipped me off to the xvda device. I'll be adding a couple other updates to the git repo, but realistically...it's just for creating an ami, and once that ami is created one should just use that ami over and over until you need a new ami. So the tool doesn't need to be /too/ sexy, just functional.