I believe it's still up in the air as to how the wireless adapter will interface...
If I remember correctly there is a FAKE image floating around that shows a USB cable which is where I think a lot of people are getting that misconception. The official fact sheet does NOT list it as a USB device which I think if it was going to be they would list it as such...
Xbox 360 Wireless Network Adapter
Designed for versatility and ease, the Xbox 360 Wireless Network Adapter enhances your Xbox 360 experience in the digital home, seamlessly synchronizing with the Xbox 360 system. You can easily transfer videos and music to your Xbox 360 system from your Media Center PC, chat with your friends, play games via Xbox Live, and more—all without the clutter of wires.
The Xbox 360 Wireless Network Adapter is highly versatile and supports 802.11b, 802.11g, and 802.11a.
The Xbox 360 Wireless Network Adapter is powered seamlessly by your Xbox 360 gaming console, so there's no external power adapter to plug in.
You can transfer high-definition video and CD-quality music from your Media Center PC to your Xbox 360 via 802.11a wireless media support.
^Unless I missed something that right there is all the info MS has released on the wireless adapter as of yet.