Last night at midnight my son bought Halo Reach and the new XBox 360 Slim. When I got up he told me he couldn't connect to the wireless router. The first thing I did was check his XBox wireless settings which looked OK. Then I had him note down the XBox wireless MAC address from advanced settings - while he did that I noticed that the 'Wired MAC address' was significantly different.
Then we went to the router and checked the MAC he had entered in the list of authorized MACs in the MAC Filtering list. Fine - he typed it in right. Then I had him view the router logs. I saw right away that the MAC address that the XBox was trying to connect with was not the 'wireless' MAC address - it was the Wired MAC address. He change the router filter MAC list to match the Wired MAC address - problem solved.
Seems like an OOPS! Hope this helps someone.