$100 will soon net you this Xbox 360 Wireless N Networking Adapter
from Microsoft and it continues the tradition of Microsoft overpricing peripherals for their console. You're paying for the 360 branding, two antenneas, and not much else more over most USB wireless adapters.
I'm, actually using a wireless N adapter with my Windows 7 Media Center upstairs and it does streaming very well to the 360 downstairs with high definition programming as well as MKV movies. It's definitely something that's worth it for that but for regular gaming or even streaming from the Internet, I don't see a reason to. I mean, most U.S. folks don't have the bandwidth to even saturate a G connection but maybe having an N adapter and router allows for a more consistent connection. I'm not too sure as I haven't done tests on that since everything I have is N now.