This manifest file shows that Windows Phone 8.1 is the name of the next build for the platform
While trying to install some Nokia exclusive apps on a non-Nokia model, someone apparently found a reference to Windows Phone 8.1 inside a manifest for an application. If legit, this would pretty much seal the deal that the next version of Windows Phone will indeed be called Windows Phone 8.1. It also means that the OS is undergoing testing this year which means it could see the light of day earlier than expected.
Last month, we told you that mobile ad network AdDuplex spotted Windows Phone 8.1 being used on the Nokia P4301 test device along with the HTC 8X and the Nokia Lumia 920. Back in March, we saw Microsoft advertising for a test engineer for Windows Phone 9.
Now that we know the the name of the next Windows Phone update, now we need to come up with the launch date. You might recall that early this month, a Nokia Lumia 920 bought from eBay contained parts of the Windows Phone Blue update including an improved calendar, and a new notifications page. Microsoft not only bought back the phone from the buyer, it also gave him a free Nokia Lumia 920. This all adds up to the possibility of a Windows Phone 8.1 update coming as soon as Q3.
source: WindowsBlogItalia (translated) via WMPoweruser