Windows Phone SDK update for Windows Phone 7.8

The Windows Phone SDK Update for Windows Phone 7.8 adds two new emulator images to your existing Windows Phone SDK installation. This update supports both the Windows Phone SDK 7.1 and the Windows Phone SDK 8.0. Using this update, you can provide the Windows Phone 8 Start screen experience in your Windows Phone 7.5 apps. You can also test how your apps will run on Windows Phone 7.8 devices.

http://www.microsoft.com/en-us/download/details.aspx?id=36474