As rumored earlier this week, Sprint is now offering prospective subscribers $100 in trade-in credit for their old iPhone to put towards a shiny new iPhone 4S. The promotion is the latest in the company's ongoing efforts to lure subscribers to its network and offset the four year, $15.5 billion deal it made to procure the iPhone earlier this year. The offer lasts from today until June 1 and is limited to new subscribers who sign up for a 2-year service agreement. The amount offered by the program is subject to the company's trade-in terms, which depending on the model and its condition could fall below or exceed the $100 amount advertised.