Heres my setup, I'm completely stumped.
university internet*------> imac
imac (using internet connection sharing) ~~~~~> xbox 360 (with a wireless adapter)
On my campus you must login to an HTML page before they will allow your machine access on the net. Its way lame, and prevents any thought of using a 360 on the network. So, to get around this I've always just used my imac to log in, then used internet connection sharing to go out from there to whatever devices I wanted to use the web without that.
For example, internet ----> imac ~~~~~> nintendo ds .....will work if I put in manual settings for the ip, dns, etc. I put in 10.0.2.1 for the gateway and dns, the ip would be anything past 10.0.2.2. That works. Nintendo Wii works the same way. Even my windows mobile phone with wifi can browse the web freely at this point. But my xbox 360 (using microsofts ridiculously expensive wireless adapter) comes up saying "not connected" when trying to connect to the network. I've put in those same settings, without prevail.
I'll totally paypal like $5 to your pocket for a working solution