You might want to treat this with a small dose of skepticism, at least until we hear official wording from Sprint itself, but according to the leaked document pictured above, Sprint will start offering iPhone trade-in credit given you grab the latest iPhone 4S. The program should kick off today and run through June 30; it should address both new and existing customers, given you sign a two-year contract and add a new line. At least $100 is what you'll get in credit for any iPhone, the exact amount probably depending on the model, condition and specs (as in internal storage). If you fancy ...