They should castrate you for even thinking of doing such a thing! There are people (like me) who are waiting it out, who didn't order a black phone for the SOLE REASON that we wanted the white one, and would only order the white one. People who waited and are ordering ONE PHONE should get first dibs on the white phone. If you ordered a black one, then you are bound by your contract, 2 years. Take it!
Sorry for sounding so blunt and frustrated about this, there have been many, many threads and discussions on "returning the black and exchanging for white". I don't think it's fair to the people who are first in line for the white iPhone when it becomes available. The first white iPhones available should ONLY go to those who WAITED. period.