Speaking of port-scanning, if you recently upgraded to MacOSX 10.2 (“Jaguar”) you may have noticed that port 631/tcp (“ipp”) is now open by default. This is for CUPS, the new print-sharing software. If you’re not on a unix-based network this is probably useless for you, so if you are paranoid like me and want to turn it off, simply edit /etc/hostconfig and look for the line: CUPS=-YES-
Change this to -NO-. Reboot, and it should be gone.