As per the announcement , the update will be available in autumn .
The Windows Phone 7.1 Beta SDK can be downloaded here ( Web Installer )
There is also an ISO file which can downloaded from the above location
The Windows Phone Developer Tools for Mango ( 7.1 ) includes
- Microsoft Windows Phone Developer Tools 7.1 (Beta)
- Windows Phone Beta Emulator
- Silverlight 4 SDK and DRT
- Microsoft XNA Game Studio 4.0 Referesh Windows Phone Extensions
- Microsoft Expression Blend SDK Preview for Windows Phone 7.1
- WCF Data Services Client for Window Phone 7.1
The Mango update includes lot of important features like Multitasking , Twitter Integration etc..
Watch the below video from Joe Belfiore , Corporate Vice President of Windows Phone Program Management who demonstrates the features in Windows Phone Mango .