This is how I got a brand new iPod Touch, which was (at the time) a generation ahead (basically a free upgrade):
1) I made a massive fuss;
2) I emphasised that I'd been here 3 times and it's costing me petrol;
3) I say, explicitly, that the newer (I had the 2G) 3G model doesn't have the dust problem;
4) He then replaced it for the 3G refurbished, and I said: "I would like a brand new one, as refurbished ones have been taken apart; therefore, dust could've gotten in to it."
5) After a bit of arguing, he said, "Would you like to talk to the manager?"
6) I said, "Yes."
7) He couldn't find the manager, and, by this time, a long queue had built up, so he gave in and gave me a new iPod Touch 3G.
This is how I would go about doing it, but in all probability, they'll just give you a iPhone 4 black refurb:
1) Make a massive fuss;
2) Emphasise that you've been here *3* times and it's costing you petrol;
3) Say, explicitly, that the iPhone white, according to internet sources, doesn't have the screen issue.
Then see how it goes from there.