OLED is a strong possibility in the future, but they have not made it to the big time yet. There are reasons for the lack of large scale use of OLEDs, including cost and reliability. They may be ready soon, but it is also possible that a newer technology may pass them by. A number of small screen devices have the OLED screens, but the numbers that would be required for the Apple products is not available yet. It could be that they will be ready for the next iPhone or the next iPad, but it will come down to several factors that no one is sure about yet.