According to a tip sent to Phone Arena, samsung developed a 5.9" UHD (746 PPI) Super AMOLED display, and will begin producing it by next August - to be adopted by the next Galaxy Note 5 phone. If true, this display will have a much higher density then the Note 4 (455 PPI) and the GS5 (432 PPI).
While this is just a tip, in January 2014 SDC already said it is planning a UHD smartphone OLED panel (back then they said that they aim to achieve 860 PPI).
As far as I know, the highest density OLED display ever shown was a 2.8" 2560x1440 AMOLED (1058 PPI!) shown by SEL a week ago. This CAAC-OS based panel was just a prototype though, and not in production yet.