Earth,
What happens with fish-eye lens is basically that the field of view captured by the camera is greater than the field of view displayed on screen.
While you're looking at the computer screen, try to imagine your whole natural field of view condensed into it. Whatever is in the corner of your right eye would be the right end of the screen, and left corner -> left end of screen. All that would be condensed into an image with a lot smaller field of view, yet it would be the same image. This is only possible with distortion.
I recently posted this about fish-eye in the Red Bull Stratos thread:
Fish eye lense does not equal round Earth. Nothing there proves anything but a fisheye lense.
Let me make this clear. It's actually very easy to verify curvature even on a fish-eye lens capture.
Pause the video on a frame where the horizon is in a horizontal position on screen, and exactly at the middle of the screen (at the midpoint of the vertical axis). Now look at the horizon. If it's curved on the frame, it's curved in reality. A straight line would appear as a straight line in a fish-eyed frame, if it's horizontal or vertical and in the middle.
Think about how a straight line arcs downward on a fish-eye image if its lower on the screen, and upward if it's higher on the screen. At the middle it's still a straight line.
Here:
http://upload.wikimedia.org/wikipedia/commons/f/fc/Vlg_shop.jpgAs you can see, there must be a line both on the horizontal and on the vertical axis, where a straight line in reality would also be a straight line on the picture. This is precisely because straight lines curve the opposite way on opposite sides of the screen.