Nope. We have white phones in my office which are over ten years old. Still nice and white. Am sure the materials used by Apple are more technologically advanced than the stuff used in the old phones.
There is a white one on display in our local apple store that is badly discolored but I don't think it is yellowing. I think it is just filthy dirty from a million people playing with it.
I'm sure apple tested it thoroughly to make sure it wont yellow.
I dont think it will go yellow, ive seen a couple at the apple store and they just get a little "dirty" look. Not yellow. Thats why I got the black one, I dont have to worry about that stuff.