A couple of findings from getting my setup working:
-If you have more than one router in the house, make sure that the device you have plugged in and bound to WAN (call it "core" or "edge" or "firewall", whatever), has UPnP enabled.
-Use DHCP on the Xbox. This will save you a lot of headache. If you so desire, use DHCP reservations on the router. (what scenarios/what reasons do people have to do static IPs on the Xbox, I'm curious)
-My particular setup uses a WRT610Nv2, DD-WRT v24-sp2 (12/19/10) mega - build 15943M NEWD-2 K2.6 Eko. However I've had plenty of success with the recommended build, 14929, and with OpenWrt as well on a Buffalo WZR-HP-600NH. I have a Linksys E4200 that only does WAP duty, and so it has UPnP disabled.
-Enable UPnP and leave "clear port forwards at startup" disabled. If you choose to clear forwards on startup, you will need to run the Test Connection procedure on your Xboxes again to make them register with the router.
-At this point, double check that you do not have any other routers on the network with UPnP enabled.
-Now, on each Xbox, go to System Settings -> Network -> (Select your connection) -> Test Xbox LIVE Connection. This will clear cached data on the console, acquire DHCP lease, verify MTU and NAT, among other things. If UPnP capability is detected the console should now register and should appear on your router.
-The first console will attempt to register 3074/udp. Subsequent consoles will grab a random high port, 20008, 20672 or some such. This is equally fine and will result in Open NAT.
-If the console reports, at the end of the Test Connection, that "You can connect to Xbox LIVE, but your settings might restrict online play", we've detected that you are behind a NAT but you won't accept unsolicited UDP (which is the reason for the port forward). This is needed to matchmake with the largest part of the service.
-If you do not care about matchmaking/online gaming on this console, you can freely ignore "Moderate NAT". The rest of the service functionality will work.
If you have further questions I am happy to answer. Very Happy
edit=Super interested to know of other multiple Xbox setups, working 100% or not, this is one area that can be difficult. UPnP should make it easier but are most folks willing to turn that on (insecure?)?