From 1up.com:

We know Microsoft's upcoming Windows Phone 7 will have access to Xbox Live, but it looks like their connectivity ambitions between the Xbox 360 and Phone 7 are even loftier than that, including not just turn-based gameplay but eventually real-time multiplayer between Phone 7 games and Xbox 360 games as well.

"So initially we're turn by turn-based," said a Microsoft representative to rgbFilter during the recent X10 event in Toronto (via Joystiq). "We are working on real-time phone-to-console, likely initially through Wi-Fi -- again, operator networks are sensitive to that. That's not on day one; day one will be turn-by-turn as well as companion-type gaming where you play a level on the phone and it may unlock a level or a weapon or some special achievement on the console game.
Full Story: 1up.com