One project means three platforms

Microsoft Demos Game Across Windows Phone 7, Xbox 360, and Windows 7

Not only is the game itself playable on all three platforms, but the session is maintained when you move from device to device: if you're playing on your Xbox and have to run out the door, you'll be on the same level when you fire it up on your Windows Phone 7 Series phone. Basically, you'll never have an excuse not to be gaming.