We often get questions about the Xbox Live service on WP7 devices and how it correlates to the Xbox Live service on Xbox 360 consoles or on Windows PC's (where it's called Games for Windows - LIVE!). Oh look! Here's one now from ericesque:
Currently, gaming on WP7 is separated into two categories - games, and Xbox Live games. The Xbox Live service on WP7 is not so incredibly different from the Games for Windows - LIVE! service on Windows PC's. Downloading games and game demos from the marketplace is present and accounted for. A great deal of the social ins and outs is present too. With Xbox Live on WP7 you can easily see what your friends are up to - be it gaming or watching a movie or listening to music.
Friend requests and game invites are also present but will often lend itself to signing in on your Xbox 360 to accept a request. Game invitations, or rather, asynchronous multiplayer isn't currently set up for anything other than Uno and Chess in Game Chest: Logic Games. Crackdown 2: Project Sunburst works in tandem with Xbox 360's version of Crackdown 2 in that unlockables will become available in each version of the games with playthroughs from each other.
So far, the majority of the Xbox Live games on WP7 aren't companions to anything available on Xbox 360 and must be purchased separately. Ilomilo is the most recent game to come out for Xbox Live Arcade that is also on WP7 and is very much the same game as its WP7 counterpart, but each must be purchased separately.
No game, as of yet, can be played 100% synchronously with other people. The tie-in that you speak of doesn't exactly exist other than that it's just an extension of the service - of the same name, on a different device. A copy of a game on PC can be completely different from the Xbox 360 version and the version of that same game on WP7 will be worlds different in format, control, fun, and especially mobility.
See our Xbox/Windows Phone 7 Guide for more info.
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Daniel Rubino is the Editor-in-chief of Windows Central, head reviewer, podcast co-host, and analyst. He has been covering Microsoft since 2007 when this site was called WMExperts (and later Windows Phone Central). His interests include Windows, laptops, next-gen computing, and for some reason, watches. Before all this tech stuff, he worked on a Ph.D. in linguistics, watched people sleep (for medical purposes!), and ran the projectors at movie theaters because it was fun.