The new Windows Phone OS lets you build user friendly and well animated applications by taking advantage of Silverlight, the platform for rich media applications.
The Microsoft Windows Phone homepage is http://www.windowsphone7series.com/ but the place for developers is at http://developer.windowsphone.com/windows-phone-7-series/.

MSDN (Microsoft Developer Network) contains parts of the final documentation. It is the best place to start reading about the Windows Phone SDK and a good starting point to begin coding.

Some useful links are listed below:

Windows Phone Development
http://msdn.microsoft.com/en-us/library/ff402535(VS.92).aspx
 
Application Platform Overview
http://msdn.microsoft.com/en-us/library/ff402531(v=VS.92).aspx
 
Class Library Reference for Windows Phone
http://msdn.microsoft.com/en-us/library/ff626516(v=VS.92).aspx
 
Code Samples for Windows Phone
http://msdn.microsoft.com/en-us/library/ff431744(v=VS.92).aspx
 
Programming Guide for Windows Phone
http://msdn.microsoft.com/en-us/library/ff402551(v=VS.92).aspx

Oana & Arthur