Microsoft recently announced that the Windows Phone 7.8 update will be coming for select Windows Phone 7.5 devices in early 2013. The update will bring a new Start Screen, updated Lock Screen, new theme colors and other features and improvements.The following video preview of Windows Phone 7.8 shows the update running on the Nokia Lumia 900, which is one of the devices which will receive the update when it is released.
We aren't certain if this is a near final build or an early preview build of Windows Phone 7.8, as Microsoft hasn't been very open about the features that they hope to offer with the update.
source - GSMArena