Sprint has yet to make money on the iPhone and probably won't be doing so any time soon, but it's apparently confident enough in its strategy to be paying for prospective customers to upgrade their phones. TechnoBuffalo has gotten word of a promotion that will apparently give current iPhone owners a minimum of $100 towards an iPhone 4S if they open a new line of service with Sprint and sign up for a two-year contract. It will apparently run between May 18th and June 30th.

That's not a bad deal, but it's also interesting as confirmation that Sprint is, as CEO Dan Hesse has insisted, "very happy" with the iPhone. It may pay up to $15.5 billion to carry the device, and this offer is yet another monetary investment in hopes of attracting the loyal, data-frugal users Hesse sees as the core of the iPhone base. In the long term, this could pay off substantially, but for right now it's put the company in some hot water with shareholders.