You can get it done at apple authorised repair sites.
If you spend well over £100 for your phone to be white your mental!
I havent got an iphone 4, or would ever want a white one, but theres plenty of people wanting to be the first with the gadgets at the moment. its one of the biggest markets right now, and if it means you spend £100 to have a white IP4, plenty of people would be queing up!