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The Windows Phone SDK Update for WP 7.8 has been released to the web and is immediately available for download via the Microsoft Download Center using the following links:
This optional update installs on top of an existing WPSDK (either 7.1 or 8.0), and provides two new emulator images that allow you to deploy and debug your app in an emulated Windows Phone 7.8 environment.
For more information, please see the announcement post on the Windows Phone Developer blog.