I've been able to hook my xbox 360 up to my airport express with security. just set it to WEP. When you put your "name" password into the apple, it converts it automatically to hexidecimal. All you need to do on your xbox is but in the exact hex password. I found a website that converts any "name" into a HEX sequence. Here is the link, let me know if it works for the new airport extreme. It should.
Just go to the bottom where is says custom WEP Key, put in whatever password you want and it will spit out the hexidecimal, use that for you XBOX password and you should be golden.
Using WEP (WEP2 couldn't take password length), i used a 13 digit password, found hex equivilant in new airport software, use connect 360, everything works great except the fact that drm won't let me play songs i paid for at itunes, (over 500 songs) Boo drm.
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http://www.csgnetwork.com/wepgeneratorcalc.html
Just go to the bottom where is says custom WEP Key, put in whatever password you want and it will spit out the hexidecimal, use that for you XBOX password and you should be golden.