AdDuplex has spotted a few devices running Windows Phone 8.1 on their ad network. The software was being mostly used on Nokia test devices, however the software was also spotted running on Nokia Lumia 920's and HTC 8X's.
Final AdDuplex stats teaser: we've seen quite a few instances of WP 8.1. Mostly on Nokia P4301 test device, but HTC 8X and Lumia 920 too.
— Alan Mendelevich (@ailon) May 4, 2013
It is currently impossible to determine what new features are included with just these stats, but it does confirm their existence. Windows Phone 8.1 is rumoured to be the equivalent to Windows Blue for Windows 8, including a new web browser and other interesting enhancements.
Windows 8.1 is expected to launch sometime later this year, with Windows Phone 8.1 launching early 2014. It's entirely possible for Windows Phone 8.1 to launch alongside Windows 8.1 if the delays are wrong.