PhoneGap is an open source mobile app development platform that allows you to create an app for Windows Phone 7 (WP7) and the same app can also be run on other app platforms. Windows Phone support was added as part of version 1.3.
Read the quick start to see how easy it to create a WP7 app.
And here are some videos on using PhoneGap with WP7: http://phonegap.com/2011/12/20/phonegap-for-windows-phone-getting-started/
And more information can be found at this blog-post: PhoneGap for WP7 dissected