Xbox on Windows phones (kinda) with Mobile GamerTag
Since it seems like ol' Microsoft is never going to get around to giving us even the slightest bit if Xbox integration with our Windows phones, at least not until presumably WM7, it'll be up to intrepid developers to fill in the gap.
Now the program is still early in development, so all you can do now is view a single Xbox gamertag along with some game stats albeit render beautifully in VGA graphics (seriously, it's very nice). You can search for any gamer and it checks with a 3rd party server to pull down the info. Not amazing but cool.
Future option include auto-updates, more resolution support and adding/maintaining friends. Grab it here.
See Microsoft, it's not too hard to throw us a bone.