Version of the Windows Phone 7.1 SDK has gone RTW get it here...
There is some (unsurprising) confusion in the community about Windows Phone versions. There have been three releases of Windows Phone OS to date 7.0, 7.0.7390 and recently 7.5 (aka mango). There have been two releases of the Windows Phone SDK 7.0 and now 7.1. You build applications for Windows Phone 7.0 and 7.0.7390 using the Windows Phone SDK 7.0. To build applications for Windows Phone 7.5 you use the Windows Phone SDK 7.1. A case of the marketing guys wanting a greater differentiation 7.0 and mango than the developers?
After installing the 7.1 SDK I tried opening one of my existing Windows Phone applications, expecting it display a dialog asking me to upgrade to 7.1, no such dialog appears, I then looked in the project properties, for something extra items in the Platform drop down (not to be found). There was a bit of a head scratch and a google search...
How to: Specify a Target Version or Upgrade Projects for Windows Phone
You have to go to the Projects menu and select "Upgrade to Windows Phone 7.1", which I think is a little unorthodox.
When creating a new Windows Phone application, a dialog is displayed allowing you to select between Windows Phone OS 7.0 and 7.1 (the developers obviously don't agree with marketing). Selecting Windows Phone OS 7.1 does indeed create you apps for Mango aka Windows Phone 7.5.
Looking past these minor inconsistencies, Mango delivers a very long list of new goodies for developers. Let the fun begin!