I bet it's for the nano to have FaceTime without including a second camera. The camera will still be on the back, but there will also be this extra screen on the back.
I delurked just to highlight this response that I didn't see anyone else quote. This sounds very likely, given:
1) The camera is deeper than the screen, and is generally harder to shoehorn in to thin designs (according to previous reports).
2) The nano camera is of course already designed for video.
3) The second facetime screen doesn't need to be high-resolution.
About the only thing against this is size issues. I can't see the same focal length being useful for both general video and video of the user holding the device, given how small the screen is. You'd have to hold it closer to you, so you'd be in extreme close up it would seem.