Disclaimer: this is pure speculation based on rumors, track record, and wishful thinking. So, no hurt feelings if it doesn't come to pass.
The way we see it, Apple methodically has been updating its entire Mac lineup with HD front facing cameras. The first to receive it was the iMac in mid-2011, quickly followed by last year's Macbook Pro lineup and subsequently the newly refreshed generation of MacBook Pros and Macbook Air; the lineup includes the 11-, 13-, and 15-inch as well as the flagship Macbook Pro with Retina display. The result is 720p high-definition video chat. The missing ingredient is the inclusion of HD FaceTime chat for iOS devices. Sure, the back of an iPhone 4/4S, iPad 2, and new iPad are technically HD cameras, and one could switch to that camera with a quick toggle, but it's still not ideal for most video chat interactions. I'm postulating that Apple could potentially introduce its first HD front-facing camera for iOS devices with the iPhone 5.