"GDIF" has written instructions on how to use your OS X box as a Software Base Station for Airport or ethernet routing.
You can find the page of instructions here:
http://homepage.mac.com/gdif/masqfirewall.html
NOTE: I don't have an Airport, and I have a hardware router, so I have not tested these myself, but they should work as expected...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010327110922863