I recently upgraded my wireless network to an Apple Time Capsule. Amazingly I couldn't get it to work with my XBOX 360, even though it worked with every other device in the house; TiVo, DVD Player, iOS devices, etc. Still unexplainable as to why it doesn't work with the 360. After reading through multiple discussion forums, google searches, and pulling out some hair, I finally got it.
Let me rephrase that. I finally FOUND the answer. It was something that was so simple, I didn't even try it.
Resetting all devices by powering them down.
UNPLUG THE POWER CORDS!
1. Unplug your XBOX 360.
2. Unplug your Time Capsule or Airport Extreme.
3. Unplug your Cable Modem or DSL Modem.
Wait a few minutes. We are waiting for them to release them from your DHCP lease and clearing any RAM that may be lingering. Sometimes leaving things unplugged really does work.
4. Plug in your Cable Modem/DSL Router.
5. Plug in your Time Capsule or Airport Extreme.
6. Plug in your XBOX 360.
7. Go into the XBOX 360 Settings and press 'Y' to have it forget the networks. It will then re-scan, and you should see your Airport device. Select it, and you are good to go!
I found a video online also stepping you through the process, essentially the same as mine.
Kudos to them for figuring it out and sharing.