If you are writing your phone applications using HTML5, you will want to check this out. PhoneGap has released an update to work with Windows Phone. What PhoneGap does is allow you to write your phone application in HTML5 and still access most of the Native API’s and wrap it in a Native application wrapper so that you can put it in the marketplace. This means you can write your application cross platform.
You can download it from here : http://phonegap.com/ or just watch the video below for more information.