To get this price you have to sign up for a new line with a two-year contract and pay $149.99 up front. Sprint will later send you a $100 American Express pre-paid gift card.
Last week, Sprint dropped the price of the iPhone from $199.99 to $149.99. That sale is still going on, but you don't need to sign up for a new line to take advantage.
Apple is matching discounts offered by retailers on the iPhone and is also planning to match this offer with a $100 in store credit if you get a new line on Sprint. Clearly the release of a new iPhone is rapidly approaching.