Windows Phone 7.8 could arrive with Rooms & Xbox Music for older devices
Windows Phone Italy said as below (translated):
The upgrade to 7.8 will be available for versions 7.0 and 7.5 shortly after the marketing of the devices Windows Phone 8.
So it seems that the Windows Phone 7.5 users could get the Rooms and Xbox Music features from Windows Phone 8 OS, apart from the new start screen of course. Also the Windows Phone 7.8 will be rolled out after the initial marketing of the first batch of the Windows Phone 8 devices is done with.