Mac OS X includes a "NetBoot" feature, whereby you can have any Mac boot from the system on any other Mac on your network. This is not an easy thing to set up, and requires installing software, setting up the root account on the host Mac, creating NetBoot share folders, fiddling with NetInfo Manager and much more.
I've posted an illustrated tutorial for implementing NetBoot here.
[kirkmc adds: Not for the faint of heart, but since it's Friday, this looks like a good weekend project for some of you. I haven't tested this.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071001144539242