Microsoft has finally RTM'd Windows Phone 8.1 GDR1 (or Update 1) internally which means a release for developers is imminent. According to The Verge, Microsoft could push out the update for those using the Preview for Developers as early as next week, or at the latest sometime in August. The update includes many improvements for OEM's and a few new ones for consumers.
Consumer changes include folders on the Start Screen, allowing users to group certain apps together much like on Android and iOS. Another rumored consumer feature is a brand new 3D UI for hardware which supports it, although we won't be seeing that UI anytime soon as hardware is yet to be available which supports it. The Store app tile is now a live tile.
Changes for OEM's include support for 1280 x 800 screen resolution, interactive phone cover support, and a number of other toggles which allows OEM's to further customize the operating system. Windows Phone 8.1 GDR1 is said to be launching as Windows Phone 8.1 Update 1, much like Windows 8.1 Update 1. It will be pushed out through the developer program first before being pushed publicly, much like Windows Phone 8.1.