Next version of Windows Phone 8.1 already in testing
Windows Phone benchmarking site WPBench has always been a good place to catch unreleased versions of Windows Phones in testing.
The latest to show up seems to be a completely new branch of the software.
While Windows Phone 8 GDR3 is OS build 8.0.10532 and Windows Phone 8.1 DP is 8.10.12397 the latest version under test is build 8.10.14130
The first update to Windows Phone 8.1 is expected to bring goodies such as double height live tiles (like on Windows 8.1) and also native folders and is expected to hit sometime in the August time frame.