In a bid to bring more iPhone customers into the fold, Sprint has started offering $100 credit towards the purchase of a new iPhone when you trade in an old iPhone from either of its competing carriers. The program began on Friday and runs through early July.

There are a number of ways to take advantage of the deal. You can take your old iPhone to a Sprint store and trade it in directly, after reserving a new iPhone 4S online. This way you get the $100 rebate instantly. You can also order your new iPhone and set up your service on Sprint’s website, then submit your old iPhone through their buyback program. If your old iPhone turns out not to be worth $100, then Sprint will make up the difference. If you go this route, the $100 credit will be posted to your account, and may be split up over 2-3 billing cycles.

The catch (not surprisingly) is that this deal excludes upgrades. That is, you can’t upgrade an existing line with a new iPhone and get the $100. You have to start a new line of service. That’s not terribly surprising, given what Sprint is trying to accomplish with this deal. Though Sprint sold a respectable number of iPhones last quarter, the company is still having major issues with profitability. In fact, the company has bet heavily on the iPhone, the point that iPhone sales are a major part of what is keeping Sprint afloat. With this deal, Sprint is plainly looking to draw in customers from other wireless carriers – i.e., AT&T and Verizon. While the company’s continued offering of unlimited data plans is a major differentiator, Sprint needs all the iPhone customers it can get. Offering $100 toward the purchase of a new iPhone (which effectively cuts the price of the 16GB iPhone 4S in half) is an excellent way to do it.

In order to get the $100 trade-in, you have to activate your new iPhone by July 3rd, and complete the trade-in process (if you go the online route) by August 14th. The deal only started on Friday, so there’s no way to know yet how well it’s working for Sprint. Unless they make a special announcement about the program’s success, we’ll probably have to wait until their next quarterly earnings report, which should come in July.