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.
See also "Xbox LIVE Extras" (messaging, scoreboard and avatar editor) and the "Xbox Companion" (Xbox remote control).
http://www.xbox.com/en-US/Live/Mobile/Home http://www.windowsphone.com/en-US/search?q=xbox