You'd think I'd never done this before. In fact, I made this exact mistake last time, too. I forgot to compile ext3 support into the kernel, which leaves me with an unbootable system.
Thankfully, I've long since learned something with Linux kernel hackery: never, ever, ever, ever remove stable kernels. So I just have to pick the other GRUB option and rebuild my kernel again. Hurray.
Stay tuned... sometime this year I might actually get a booting 2.6 box [rolleyes]