Download the Windows Phone Developer Tools (WPDT) 7.1 Beta and experiment with the latest tools release for Windows Phone. WPDT 7.1 Beta provides the tooling necessary to target the upcoming Windows Phone OS release (codenamed “Mango”) in addition to productivity enhancements such as an app profiler and an improved emulator.

Developers interested in evaluating the beta should install it on a secondary machine. It is recommended that you continue to use the existing WPDT 7.0 tools to develop and test Windows Phone apps that target Marketplace customers running Windows Phone OS 7.0.

and for more resources, please visit http://create.msdn.com/en-us/resources/downloads  

Read this blog from Windows Phone team as well  Developer News: Beta Mango Tools Available Today , to know about the features. I was listening to the live webcast happened yesterday(announcement webcast video available here), and we have around 500 new features been introduced in Mango release. All the features been demoed yesterday in live cast.

Another interesting link you could try reading would be a previous press release from microsoft about mango.

http://www.microsoft.com/presspass/features/2011/may11/05-24WindowsPhone.mspx

Andy Lees: Windows Phone ‘Mango’ Press Conference

Download from Microsoft Download Center