Applications for Windows Phone 7 Series can be developed using an enhanced version of Silverlight 3 or XNA, a tool chain to develop Xbox games. The also announced Silverlight 4 RC does not support development for Windows Phone 7 Series as of yet.
You can download the free Windows Phone 7 Series development tools beta version:
- Expression Blend for Windows Phone for Silverlight development
- Microsoft Visual Studio 2010 Express for Windows Phone (preview version)
- Windows Phone Emulator
- XNA Game Studio 4.0 CTP