When Apple showed the front-facing camera on the iPhone 4 at WWDC, it seemed it might be accessible to FaceTime only. Now, with the iPhone 4 out less than a week, there is already at least one third party app in the App Store that uses it.
New app Mirror with Front Camera Support uses this camera to let iPhone 4 owners see themselves in any one of 4 decorative mirror frames. You can use the app to check your hair, or use the iPhone's screen capture ability to make a quick portrait of yourself.
As you can see above, the app does have it's obvious limitations. The popup says the app "works best" on iPhone 4, but in fact it is required. Due to this fact, it has already gotten one negative review from someone who tried to use the app on their older model iPhone. Since it captures images with the front-facing camera, it obviously will not work on devices that do not have one.
While this is not the most sophisticated or useful app, it does demonstrate how the front-facing camera can be used for things other than just FaceTime. Hopefully the app will get an update some time in the near future to allow saving and sharing images more easily.