Carriers in China were only recently given the go-ahead to start selling the iPhone 4S, but they’ve already figured out how to use the handset to drive business. For example, China Unicom has quite the promotion involving the new device.

Bloomberg is reporting this morning that the world’s third largest cell phone provider is about to launch a campaign that will allow customers to pick up Apple’s latest smartphone for free, as long as they’re ok with a long-term contract…

The promotion, which starts January 13th, will allows users to pick up a free iPhone 4S for signing a multi-year contract with the carrier. Subscribers willing to sign a two-year deal will get the 16GB model, and those that sign a three-year deal the 32GB.

Let’s do some quick math. To get the free iPhone 4S, customers must sign up for a monthly plan starting at 286 yuan, or $45. If customers sign a two year contract at that price, they’ll end up paying China Unicom $1080 over the course of the contract. Now if we know that the 16GB iPhone 4S costs $650, we can gather that the carrier still makes roughly $400 per “free” handset. That is, assuming the customers continue to pay their bills.

While we love the sound of a free iPhone 4S, analysts believe that the move is far too aggressive. They’re thinking that all of the new customers will add weight to its already struggling data network, which will obviously cost more money to maintain.