Amazon has the official Microsoft wireless network adapter for sale... This snaps on to the back of your 360 and plugs into the USB port on the back. If you are looking to stream movies wirelessly via Windows Media Center or Vista, this thing supports the higher dedicated bandwidth mode that you need.
Normally retails for $99.99. Grab it for $89.99 with FREE super saver shipping and no tax!
Check it out: Amazon - Xbox 360 Wireless Network Adapter