from the day you exchange your iPhone 4 there will be a freeze on your iPhone upgrade eligibility for one month, meaning that you have to wait a month before buying an iPhone on your line, avoiding this whole cheating the system idea. It may or may not cover a family plan loop hole
I returned my black iphone just this past week (Thursday). I have already regained my eligibility according to ATTs website. It would allow me to repurchase the same black 32gb phone at the discounted rate if I wanted to. I am, however, going to wait for the white.