I came across this little bit of info over at The Creative Guy. His story talks about this built in feature of OS X 10.4 (tiger). You can access these settings by going to your “sharing” pref pane, and clicking the “advanced” button.
“…Essentially, Stealth Mode hides your Mac from a ping so that anyone who is looking at hacking in to your Mac will have a more difficult time doing it… even if they have your IP address.”
Great, I just turned on Stealth Mode :D .