I’ve now figured out how to keep my old Apple Airport happy with the new AT&T setup. The way things were set up before, the Airport was plugged into my Time Warner cable modem, and thus was directly exposed to the ‘net. I had one port forward set up, to a RHL virtual machine running in Parallels on my iMac. This RHL box runs a WAIS server (freeWAIS-sf) with 20 years of postings to a flyfishing listserver indexed for searching. By putting the Airport into the “Enhanced DMZ” of the 2wire router, it shares the external IP of the native router, and the port forward from the ‘net to the VM works fine. Updated the Dynamic DNS link and it’s good to go!
By the way, the marketing foo from AT&T says that they only support 10 concurrent devices, but the 2wire box was set up to issue 200 DHCP leases. That was worrying me when I heard about the limit of “10 devices” but it looks like it’s not a hard limit. Also, AT&T does not try to keep you from pwning your own router, you can go in and make changes to the whole “home network” IP configuration. If you are curious about the box, the model I have (3800) is apparently very much like the 2700. The manual for the 2700 is here.