Yes, Phono-1 MkIII is in the works. This revision is driven by necessity to change sheet metal supplier - the chassis will be the only new part in the MkIII. Electronics and power supply are not affected.
There will be no change in performance, and price is expected to remain $299 as Phono-1 enters the 6th year on the market (wow - I just figured that out - time really flies!).
Phono-1 MkIII is expected to be available in November.