I’m pleased to announce that the beta release of the Windows Phone Developer Tools that support Mango will be available for download today: Matt Bencke General Manager, Windows Phone Developer and Marketplace Experiences
Note: I took above image and statement from http://windowsteamblog.com
This was one of the most waited releases. You can download it from here.
On successfully installed Mango on your system, you will get below project templates under Silverlight for Windows Phone tab
You can choose target window version as well. Mango version is 7.1 
New Features are as follows
1. New Back ground processing.
I am going to love this feature most. Now you can download a file in background while other application is running on foreground. It was not the case with WebClient
2. New Profiler and Emulator for testing
3. Location support
4. Silverlight 4.0 and integration of XNA with Silverlight 4.0 for better gaming experience
5. IE 9 as browser
6. Socket for communication
7. Local SQL for Structured Data structure
8. Fast Application switching etc
If you want to work with demo code sample, You can go here
For learning in detail go here
