If you’re an iPhone customer considering jumping ship to Sprint, the Now Network’s upcoming promo just might be enough to influence your decision. A trusted source has shared with us that starting on May 18th Sprint will begin offering a minimum of $100 on any iPhone trade-ins towards an iPhone 4S. This offer is available to new and existing customers alike, but it does require a new line of service and a two-year commitment. So, if you’ve got an old iPhone 3G or 3GS collecting dust, and you’re ready to make a move, then you may want to give this offer a whirl, but be sure to act fast, as it is slated to end on June 30th.