Better late than never, we have a workaround fix for the compatibility issues we reported earlier this month regarding the new Airport Extreme Base Station and Xbox 360. Go to your Airport Utility, then Option-Click the radio setting button to reveal a drop down menu, from there, select "b/g compatible", not "n (b/g compatible)". That takes away the benefits of an 802.11 n network, but it gets the job done with 54 Mbps.
Next up is to Option-Click the button for encryption setting and select "WPA Personal". In DHCP settings under Internet, using the 360's MAC address, reserve an IP so it stays consistent. Finally, in your NAT options, select the 360's IP address as the "Default Host" to prevent any Live functionality issues, and restart your Extreme Base Station. Once you go to the network settings on your System blade, you should see your WPA-encrypted network show up. So your 360 and Airport Extreme can work together with encryption, although you kind of neuter the Base Station in the process.