- No Second Camera - The most central reason that FaceTime won't come to the 3GS or 3G is that those models have only one camera on the back, while FaceTime requires a user-facing camera. The user-facing camera is needed to take video of you while also allowing you to see the person you're calling on the screen. While the 3GS or 3G's back camera could take video of you fine, you wouldn't be able to see the person you're talking to--thus negating the idea of a video chat.
- No Software - Because their hardware doesn't support FaceTime, Apple doesn't make the FaceTime software available for either of these models, even with iOS 4 or higher installed.
Works via Jailbreak/Cydia
That said, the software issue can be overcome by jailbreaking your phone and installing various third-party programs via the Cydia App Store. One such program is FaceIt-3GS. It's important to remember, though, that jailbreaking your phone can void your warranty or cause other problems and should only be done by the tech savvy comfortable taking risks.
On 3G Cellular Networks
Just like FaceTime won't work on the iPhone 3G, it also won't work on the other kind of 3G: the cellular data network that the iPhone and iPad can connect to. For now at least, FaceTime only works on Wi-Fi networks.
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