Here's a good news for Windows Phone developers. Windows Phone 7.8 SDK is just announced and is an optional update. Yes, that's right. The SDK mainly adds two new Windows Phone 7.8 emulator images to existing SDK installation with variation of RAM (256/512).
The update complies both with Windows Phone SDK 7.1 as well as Windows Phone SDK 8.0 and provides new start screen (three variations of tile sizes) experience as introduced in Windows Phone 8.0. You can read more details of the SDK here, http://blogs.windows.com/windows_phone/b/wpdev/archive/2013/01/22/now-available-windows-phone-sdk-update-for-7-8.aspx
This should also be kept in consideration that Microsoft has already confirmed that update for 7.8 will be rolled out to consumer devices starting January 31, 2013.