For those bothered by this who would like to be able to prevent someone from doing this, you may want to use the Open Firmware password application. The page on Apple's support site to download it is found here. Installing this prevents starting up in single user mode, resetting the PRAM, and other things insanely useful when stuff goes wrong. Be careful, though, because if you forget this password, you're screwed.

Yes you are screwed if you forget the Open Firmware Password, however, a quick trip to your Apple Authorized Service Provider can fix it for a small fee and lecture!

You can save yourself a trip to an Authorized Apple service center if you change the physical configuration of the machine (i.e. remove/add RAM). This forces a reset of any password applied to the Open Firmware. This is what the linked KB article says.