I have written a small program called natpmpmap (8KB download) which makes it possible to change the NAT port mapping settings of modern AirPort Base Stations. As you may know, AirPort's Admin Utility doesn't allow one to set UDP port mappings. That brings a lot of headache for those of us who want to deploy, for example, QuickTime Streaming Server.
The program is very simple to use, and it's a Universal build with source code included. Feel free to use it, to share it, to change it. Probably in the near future, if I'm not too busy, I'll write a standalone daemon with launchd support and nice GUI configurator.
[robg adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006111813554987