I was curious about that as well, and I found the answer on page 60 of the Designing Airport Extreme Networks manual (PDF). Looks pretty straightforward.
Thanks mrogers. It does look pretty easy, but unfortunately, it's just port forwarding and not triggering, which I've recently discovered is handy for BitTorrent and multiple machines on the network (it lets the BT incoming traffic go to the machine that is running BT instead of to a static IP set in the forwarding table). This alleviated my issues with having World of Warcraft on multiple machines, since it uses BT to patch.
Oh well - perhaps Apple will add "advanced" stuff like that to the Extreme at some point.
Mike