OLED is still too expensive and in short supply from a production standpoint. Apple gets their displays from Samsung and LG. LG uses all their capacity for larger form factor Tvs and monitors. An OLED manufacturer would have to be willing to do production for these smaller devices and it would require a premium. I think that premium could easily be in the 200$ range for a device like the ipad. I don't know that people would want to pay it. And IPS LCD is REALLY good at this point.
Also, don't forget that REPAIRS would all be much more expensive, either user or apple provided repairs.
I think we won't see OLED next year for these devices yet, could be easily 2-3 years out - if at all.