It was an available option under OS 9, I'm almost sure...
I've noticed when I put my iBook or TiBook to sleep, it's ethernet connection remains when I look at my hub. I wonder if that's significant at all.
I used No-IP in the past to remotely ssh into my TiBook and it worked great, but having my machine wait in a sleep state would be pretty cool too. I wonder how one would go about putting it back to sleep again?