It wasn’t too long ago that Sprint’s Dan Hesse remarked that while getting the iPhone onto their network did cost the carrier a pretty penny, he believed that they would be able to start profiting from it in a few years’ time. Perhaps look to speed up that process is an upcoming promotion by Sprint that supposedly will launch on the 18th of May. This is courtesy of the folks at TechnoBuffalo whose source revealed to them that this will be a trade-in promotion, whereby current iPhone owners can trade-in their devices and get at least $100 of credit towards the purchase of an iPhone 4S. Of course this would mean that users would have to sign on to a new two-year contract, but if you’re currently not on Sprint’s network and you’re thinking of upgrading your iPhone 3GS to the iPhone 4S, we guess this promotion does sound like a pretty good deal. However until the promotion has officially been announced, we suggest you not get your hopes up just yet and be sure to stay tuned for updates!