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.