Why Was The White iPhone 4 Delayed?
The white iPhone 4 was delayed due to problems with two components: the camera flash and the proximity sensor. The combination of the flash and the white back of the iPhone resulted in very low-quality photos. The proximity sensor (which senses when you are holding the phone up to your ear) also had problems caused by the white case.
The white iPad 2 does not have these same problems because it lacks a camera flash and doesn't have a proximity sensor.