Sure enough, Microsoft will release anytime (next year) Windows Phone 8.1, which is expected to follow the latest Windows Phone 8 GDR3 release and is even said to somehow unify the Windows RT and Windows Phone 8 codebase - in some way. However, as a recently published screenshot shows now, Microsoft seems to be already beyond planning and is dog-food'ng it already ("dog food" is Microsoft's term for Alpha and early Beta builds and tests). The published screenshot from a Windows Phone app called "FB Pages Manager WP" shows firmware version numbers greater today's known Windows Phone 8.0.xxx, showing several Windows Phone version from 8.1.12115.0 up to Windows Phone 8.10.12236.0.
Microsoft has yet neither spilled any beans what Windows Phone 8.1 might be about nor when we can expect it but it's good to see that it's already working on it and that Microsoft seems to make some real progress, at least that's what the different build numbers indicate.
Cheers ~ Arne