Microsoft announced that the "new Windows Phone 7 SDK features Bing Maps as its default map control," and that "Bing Maps is now free" for you to create consumer-facing mobile apps on any platform. If you're new to Windows Phone development, it's super-easy -- see the video below about reports of 3-hour-dev cycles vs. two weeks or more for similar-functioning Android builds.
Of course you can download Windows Phone 7 SDK, or check out MSDN's resources to learn more about new Bing Maps Control for Windows Phone 7. More free options are available for other popular mobile platforms through Bing Maps REST Services. It's easy to get started, check out developer resources to see all the free ways you can create innovative mapping applications, fast.