“These two emulator images should enable you to fully test how your Windows Phone 7.5 app’s Live Tiles will look and behave when they are run on a device running Windows Phone 7.8,” Microsoft writes.
Most importantly, it’s another indicator that the update itself is finally, finally coming. Windows Phone 7.8 is the last update to WP 7.x-based devices like the Nokia Lumia 900. It brings only a few features, most notably small-sized live tiles to the home screen, and Microsoft has earlier vaguely said that it’d arrive in “early 2013.”
Microsoft also makes mention of new phones on 7.8 “beginning to ship,” which is good news.
source: Windows Phone Developer Blog