This webpage speaks of how the command nvram, located in the /usr/sbin directory, can be used to "list and set open firmware settings," including "specify[ing] your own settings. For example, you could create a variable called asset_tag and keep the asset inventory tag of that particular computer in the firmware."
This particular tip was thought of by the webpage's author as a hint for people running computer labs, but it could be used by other Mac OS X users, I'm sure.
I'd really, really pause and make sure you know what you're doing before goofing around with your computer's firmware, though ... talk about playing with dynamite!
[Editor's note: I'll echo the sentiment -- this is published here mainly in the interest of completeness. Do NOT mess around with Open Firmware unless you really really know what you're doing!]

