A very simply way to disable single-user mode is to edit /etc/rc.boot (as root or with sudo) and in the section that prints "file system mounted read-only", simply add a line that says reboot Now, anytime the computer is booted in single-user mode, it simply reboots itself. This is totally non-bypassable because Apple disables any usable keys in the begining of the rc files.
I have mine set to do fsck;fsck;reboot because the only reason I boot single-user is to do fsck, and it also keeps the system secure.
If you're confused about what I mean, then you probably shouldn't use this hint.
[Editor's note: Although I haven't tested this hint, I looked through rc.boot, and it appears the section of the file you need to change actually reads ConsoleMessage "Root device is mounted read-only", and then there are a couple more lines of ConsoleMessage output, then an "else." You should insert the "reboot" before the "else," I believe (someone please correct me if I'm wrong).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021111055147112