We tried rebooting, we tried zapping the PRAM, and we both booted into single user mode and booted the computer in target disk mode, attached to my laptop, and performed a disk check on the computer. No problems with the disk. The machine just wasn't booting.
After throwing up my hands and proclaiming there to be some odd hardware failure, it suddenly hit me: maybe the login window preferences were borked. We booted into single user mode again (reboot and hold down the command s button). I ran all the commands that you have to run in order to write to the disk (
/sbin/fsck -yfollowed by
/sbin/mount -wu /). Then I ran:
mv com.apple.loginwindow.plist BACKUPcomappleloginwindowplist
This makes a backup copy of the preference file associated with the login window and erases the original file in the process. After I was done, I typed
rebootat the prompt and hit the return key. OS X rebooted and was forced to create a new preference file for the login window, thus fixing the problem. Huzzah!