AdDuplex has one more May teaser before their full report. Apparently their ad network has noticed some Windows Phone 8.1 handsets being tested, mainly on the Nokia P4301 test device, but also on the Nokia Lumia 920 and the HTC 8X.
Of course what exactly Windows Phone 8.1 is has not been revealed. It is rumoured to be the Blue version of Windows Phone, with IE11, but it could as easily be a GDR 2 version of the OS, given the small bump in the OS number.
Lets hope the OS being tested now is a sign that rumours of its delay are wrong, and we will indeed see a substantial update in fall as we have come to expect.