In fact your windows 8 account is linked to your xbox 360 account from the start, although I'm not sure if you can use some of the features (like Party Chat). Games for Windows Live allows you to use your live account, yet, even if you are a gold account member, you can't use features exclusive to gold, like party chat, which is really a shame, especially when the 360 still doesn't have skype support.
I'm really into home theater, and while I like gaming on my front projector with a mouse and keyboard, it's still pretty inconvenient and cumbersome, even if you want to use a gamepad, you still need it to use the menus and interface to use it. I'd like to boot my computer, start up steam through metro, and fire up Team Fortress 2 or Battlefield 3, all through a wireless 360 controller, as well as using a 360 headset to smack talk at people just like I can with a normal 360, only on a cinemascope screen at 60FPS maxed out. Maybe party chat with some xbox buddies while I travel through Skyrim on my laptop would be nice too.
Speaking of which, many the folks at Valve like Gabe Newell, do not like Windows 8, where Newell called it a "Catastrophe" complaining about the closed nature of Windows 8. Microsoft has planned to adopt the Apple policy of apps that they take about 20-30% from apps being sold on their store.
I bring this up since Steam is a pretty big deal to me, and Valve is also planning on the "Steam Big Picture" interface intended for home theater gaming. I'm hoping that Steam Big picture will work well with Metro because not using it might be counter productive with Big Picture.