I wonder why MS hasn't leveraged the fairly successful Xbox Live social network (that I know about, anyhow) to build a solid fanbase for their phone offering. Score-watching, social-sharing, and franchise related minigames with exclusive content or game items seem a natural extension of the XBL ecosystem and would build brand loyalty with a demographic that has a particularly high appetite for consumer tech as well as a fairly high disposable income.
They have. Kinectimals, Fable, Crackdown and Need For Speed all have Windows Phone mini games/apps that allow you to earn unique achievements and items. Halo has a 'dashboard' app, but not a game, afaik.
See also "Xbox LIVE Extras" (messaging, scoreboard and avatar editor) and the "Xbox Companion" (Xbox remote control).
Those games are mostly old as sin, with the newest (aside from Kinectimals) predating Windows Phone 7 by a year. No surprise they aren't getting any attention.
