As noted in the linked Ars Technica article, Apple messed up and left the IPv6 firewall off. Oops.
Out of the box, the router will connect you to the IPv6 Internet using an automatically configured tunnel. This means putting IPv6 packets put inside regular IPv4 packets. Those of you who really want to test IPv6 (you know who you are) are better off manually configuring a tunnel to your ISP or a tunnel broker, this is faster. If you don’t want IPv6 and don’t want to turn it off on all MacOS X and Windows Vista systems connected to the AirPort Extreme, you can select “Link-local only” as the IPv6 mode. If you leave IPv6 turned on, you may want to select “Block incoming IPv6 connections” to turn on the IPv6 firewall or your network is wide open over IPv6, even if it’s firewalled over IPv4.