I got this problem today - finally can connect to xbox live (really was apartment issue so once move terus can).
using d-link dsl-2640t (http://www.dlink.com.my/products/?idproduct=165) a router/modem

i try setting static ip cannot.
it connects flawlessly via auto

whichever connects first has priority

if xbox first, then laptop (and anything else) is shutdown
if laptop connect, only xbox cannot while other wireless device is ok (htc, wii, 2 netbooks)

settings issue? is very weird and hope to solve soon tq