Only a problem with the XBOX. All my other devices connect just fine.
I've tried Mixed, Mixed B/G, Mixed N/G, auto channel, channels 1/9/11, wpa 2, wpa, wep, and no security. All in combination with each other. Still was not connecting to the router.
Switching to the "User Friendly" Buffalo firmware allowed the Xbox to connect right away so I'm guessing there is some setting that I'm missing in the dd-wrt firmware that is preventing it from connecting. Unfortantly I can't stay on the buffalo firmware because it is causing issues with my other networked devices.
edit: And this was using the latest firmware posted on the support site (v24sp2-18795)