Windows Phone 7.8 begins to be shipped with new phones and the update for 7.5 devices should start been deployed “over the air” very soon. That is the consumer part. For the developer, the good news was published today by Microsoft, the Windows Phone SDK Update for 7.8 was released this morning. You will be able to use some of the new features of Windows Phone 8 in your application that target Windows Phone 7.5.
You can install this SDK on Windows 7 and Windows 8 and it will add two new Emulators: Windows Phone 7.8 512 MB emulator and Windows Phone 7.8 256MB emulator. You’ll still be able to test your apps on Windows Phone 7.1 (or 8.0) because this new SDK is installed on top of the previous installs. There is no new API provided with this SDK, only the two new emulators.
The main (if not only) new capability that you can take advantage of in your app is the new live tile option:
The primary tile supports now 3 different sizes (like for Windows Phone 8), small, medium and large.
Download the Windows Phone SDKs