I've got an xbox plugged into my AE, and I've not had to do any port-forwarding to make it work.
Did you set anything in the 'Access Control' part of your AE admin setup? If you've tried to do mac filtering etc, you'll need to add the xbox wireless controller to that. (Without it, it may well get a connection, but not be given an IP etc)
Ok - sounds like the XB isn't getting an IP address.
Is the AE new too, or have you been using it with the cable modem for something else? It just sounds like DHCP isn't turned on one of those fellas to me.
If you didn't do anything out of the ordinary when you setup the AE, I'd expect it to be running DHCP.
If you use the Airport Admin utility, select your airport and then click on the 'Network' section you should see the 'Distribute IP address' box ticked. If it is, then the AE is running DHCP.
If that's the case, then click on the 'Access Control' section, and that should be empty (if it isn't then this would be the problem).
If the above two points are ok (DHCP is on and Access Control is empty), then I'm a bit stumped as to why the XB-Wifi is connecting, but not passing an IP address to the Xbox. I've never used one before, but I'd expect it should make itself transparent to the network in most respects.
If the AE is not distributing IP addresses, I think you'll need to turn that on, as looking at the comcast info, I doubt the cable modem is doing DHCP.
At the moment, your xbox isn't getting an IP address, so there's no messages to get blocked by closed ports.
The AE doesn't have a firewall, and without being sure, I doubt the cable modem does either. So there should be no ports that need to be opened or forwarded. (I've never had to for my xbox live with halo 2 or anything else)
I have a Wireless Adapter hooked up to my Airport Basestation, and I vaguely recall reseting the cable modem when I added the M$ adapter. I suppose you've already tried this, but if not, it might help. Good luck! - j
ps. I do remember having to select an available port connection when connecting my PC to the basestation as well - I think the Xbox adapter needed that too. The reason you are getting an assigned IP and good connection meter is because the Adapter can communicate with the basestation - but perhaps not outside of that.