Turns out that if you update the kernel with an external USB disk plugged
in, grub gets hosed. When I originally did the system yum update I had a
disk plugged in, that broke it the first time. I still had that external
disk plugged in for each attempt at rescue/grub2 reinstall. Finally it
dawned on me to try it without the external disk, and booting from the
install cd, rescue mode, and reinstalling grub fixed it all.
Weird bug.