If you a re going to make an app for either Windows Phone 7 or Xbox Live it seems there is no reason not to make it for both.
This seems to be the philosophy of Sudoku3D, who created their eponymous game to play on both platforms. Sudoku3D is a new variation of the well known game Sudoku. Unlike others Sudoku3D uses 16 unique numbers and is a single puzzle on a surface of a cube.
The game clearly works pretty smoothly ( a word I use often) on a real device, and in all likelihood will be ready for Marketplace by the time of the launch.
Follow Sukoku3D on their facebook page here.