Are you an old iPhone owner (on any carrier) who's been wanting to upgrade to the 4S but has always found the smartphone's price just slightly out of your reach, budget-wise? Well, today's your lucky day. Sprint is offering a limited time deal that shaves $100 off an iPhone 4S via a special, trade-in promotion.

There are two ways to redeem the offer: either in-store or online. For the in-store method, you'd have to reserve an iPhone 4S unit at www.sprint.com, bring in your old iPhone, and save $100 instantly when you activate a new line of service.

To redeem the offer online, you would hit up http://sprint.com/iPhone to buy your iPhone 4S through the carrier's online store and open a new line of service. Then after you receive your new iPhone, get on http://sprint.com/buyback and click on Trade in my device to receive a shipping label via email. Sprint has a caveat here in that if your Buyback value is less than $100, they'll make up the difference with an extra account credit. Additionally, you'd have to activate your phone by July 3rd and execute the trade-in by August 14th.

Then you wait for the credit to appear on your bill within 2 to 3 billing cycles. Sprint says this credit may be split into 2 payments, and you should wait for the full 3-month billing cycle before you raise a ruckus about not receiving the full $100 credit.

Given the hassles of the latter method, it seems like you're much better off with the in-store trade-in route. Just one last (rather big) word of caution that both ways exclude upgrades -- so we suggest that you take that into account before you jump to avail of this offer.

Sprint's promo officially goes into effect today, May 18th.