Ok, forget the one Microsoft/TiVo/whoever sells. You don't need it.
What you need is what is called a bridge. It connects wired boxes via a plain old cable to a WiFi network.
So anything that has a network plug can plug into the bridge without buying that special expensive adapter for your TiVo/Xbox/Xbox 360/etc.
Something like this is what you want, and if you want to plug multiple devices in, get a switch or hub.
Sometimes, they have a switch built in like this one:
That is the one I have and it's ROCK SOLID. Not the cheapest, but recommended. You can plug 4 devices into the switch on it. So you saved $20 by not buying the official Xbox 360 adapter, and now you can plug EVERYTHING in your entertainment center into your network.
I've gotten over $200 in cash back from them at places I would have shopped anyway! Ebay, Staples, more! I put it at the top because they ROCK. Basically, you get a kickback from Ebates when buy.com or whoever pays them. Win-win. If you like throwing money away, don't use them!