Image from NYTimes
Apple explained this a while back. They mentioned that preserving the quality of the plastic was difficult. They had to look at how it would feel and look over time. White plastic easily get grungy and nasty. It can also yellow. Especially when it comes to a device like a phone. It's something we carry around all day. Hands get dirty, and that in turn makes your phone all kinds of dirty. That is unless you plan on washing your hands every time you plan to reach in your pocket.
There were plenty of rumors as to why the handset was delayed a few times. Many pointed to issues with the camera and with the proximity sensor, both of which are slightly modified for the newer iPhone's design.
NYTimes' Nick Bilton took a look at two white iPhone 4 models. One was the official release from Apple, the other was one of those first-run models from a factory in China. Bilton found there there is definite discoloration on the unofficial iPhone. The proximity sensor is also different, indicating that Apple definitely worked on some variations in hope of improving the design. This is because the lighter color of the white iPhone makes the proximity sensor more sensitive. At least that's what the speculation was with some of the first rumors as to why Apple delayed the device. It makes perfect sense though.