In short, eject the SD card from your Pi, mount it somewhere, and add "single" to the end of cmdline.txt. I was able to stick the card in my Mac and edit the file. Eject the card from that computer, stick it back in the Pi and boot it up. As is typical on Linux boxes it will boot to single user mode and prompt you for the root password.
Once you've booted up and fixed whatever needs fixing you can edit /boot/cmdline.txt on a running Pi and remove the "single" keyword.