The developer tools for Windows Phone Mango were updated to Beta 2 a few days ago. You can find the bits here,
Web Installer – http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=26648
ISO file – http://go.microsoft.com/fwlink/?LinkID=220415
You can read this WindowsPhoneGeek article on installation instructions.
On June 29th, Microsoft also released Mango for developers,
I am still waiting for my invite. Looks like they are sending out invites starting from the list of supported countries, which unfortunately, India is not a part of. I’ll write when I get the invite and let you know how the installation goes on my HD7.