I just got an iPhone 4s from my local Sprint store. Everything works perfectly, except for the front-facing camera. When I use any camera app (the stock one, Camera+, or Instagram) and click the icon to "flip" to the front camera, it wigs out and glitches, giving me weird lines and darkness. Sometimes it comes up with a full color, normal image, but most of the time it either shifts down to a grainy picture or simply freezes up. Switching back and forth between front and back cameras has mixed results, but nothing stable. I tried using FaceTime with a friend, and I had this same problem (glitched, frozen screen with weird lines) for about 1 minute, then it switched back to full color, normal picture.
So the problem seems very intermittent, and I have no idea how to stabilize it. I plan to go into the Sprint store tomorrow and show them, and hopefully either get a fix, or a new phone. But if anyone has had this issue (I see some posts about it already, but most of them are from last year) and knows how to fix it, please let me know!
p.s. I have not done a factory reset yet. But I have read other posts where people say this had no effect on their problem anyway.
Those are Apple's recommended troubleshooting steps, so there's really not much more to recommend. If all the steps, including restoring as a NEW device fail to resolve your problem, then you'll need to bring your phone into Apple for evaluation.