A job posting on Microsoft's site for a Principal Program Manager specifically asks the successful applicant to "drive the platform and bring Xbox LIVE enabled games to Windows Mobile". That's the part we knew.
Some new stuff, however, comes in how they'll be doing this, saying "This person will focus specifically on what makes gaming experiences "LIVE Enabled" through aspects such as avatar integration, social interactions, and multi-screen experiences".
So...your avatar can hang out on your 360 and your phone. That was a given. The "multi-screen" part is interesting, though. Remember how the GameCube and GBA could link up and share game info? Like, how in Wind Waker you could get little maps and bonus info on the Game Boy Advance's screen? This might be like that. Only, people might actually use it.