One of the new features coming tomorrow with iOS 5 is Apple’s iMessage service. Poised to replace costly SMS, the new feature is being seen as a threat to carriers such as AT&T, Sprint, and Verizon, according to The New York Times.

First unveiled in June, iMessage works through the existing Message app, delivering real-time messages to others that can include photos and videos. The only requirement is that the other person must also be using an iDevice with iOS 5 installed.

Unlike SMS, iMessages are free and unlimited. As such, each message transmits over a carriers’ data networks and the Internet, much like e-mail, and not through cellular networks.

Naturally, the more customers begin using iMessages, the less likely they will use SMS, which run anywhere from 10 to 20 cents per message or are purchased through carriers as a monthly fee.

According to The New York Times:

Analysts say Apple is trying to duplicate the success of services like BlackBerry Messenger, or BBM, a free application for BlackBerry smartphone owners that lets them send messages back and forth as in an instant-messaging conversation. It has engendered loyalty among BlackBerry users and has kept some from switching to an Apple or Android device.

“BBM is the stickiest feature of the BlackBerry experience, even more than e-mail,” said Roger Entner, an analyst at Recon Analytics who follows the wireless industry. “Once you have that, you are considerably less likely to switch away from the consumer experience. IMessage makes the whole iOS universe more valuable.”

Best of all, iMessage is seemless. On iDevices with iOS 5, iMessage is automatically turned on via the Settings app under Messages. On the iPhone, any message that cannot be sent via iMessage is automatically sent using a carriers’ existing SMS service.

It will certainly be interesting to see how receptive iDevice owners will be to iMessage and whether it actually hurts carriers in the long run. We’ll keep you updated.