You really do learn something new every day. Today, it was that EFI Macs can be set from the command line to boot into FireWire target mode on their next reboot (Open Firmware macs can do it, too, but it's more complicated). Nifty. Why would you want to do this? A couple of reasons. One, if you, like me, tend to be doing two, or three, or ten, things at once, holding down keys at startup can be a pain. I often hit 'reboot,' figure I have enough time to finish off a task on another machine, get caught up in the new task, and miss the window of opportunity to start the first machine in target disk mode, switch the startup disk, or whatever it was I wanted to reboot for. with the command line, it's "no muss, no fuss," just:
sudo nvram target-mode=1
That will set the machine to boot in target disk mode at it's next start-up. It only works for the next start-up, and can't be unset. That's a actually a bit of a pain: habitual command line users expect that '1' will toggle a behavior, and '0' will untoggle it, but that isn't the case here. Any value, even zero will work.
Another potential use is for a disk you suspect is corrupt, or to unstick a frozen system. Often, you can ssh in from another machine (assuming you have remote login turned on) even when finder crashes and a machine appears to be frozen. Just toggle target mode, issue a quick sudo shutdown -r now, and you can plug the offending machine into another computer and run your diagnostics. That, and it's a cool party trick.
Thanks to Matt for pointing this out.