It's not really my engineering field, but...
For many years, CCFL was the technology of choice. Lower power than anything else, including LED. I remember changing the light tube on an HP Jornada handheld PPC that went out on me. Looked just like a very thin flourescent lamp, which it is.
However, the iPhone LCD assembly looks way too thin to use CCFL. And almost everyone has used LED backlights for smartphones for about five years now. So I go with LED (or a small probability of EL panel).
(To those who wonder: LED backlights are new on larger displays, not on small. It's only recently that using dozens of LEDs to backlight a large LCD panel became as power efficient as using one or more CCFLs.)