Left: original iPhone, Right: new iPhone?
Forum user mrat93 found an interesting image of the iPhone in a file called "iPhone.icns" that accompanies the iPhone SDK. Amongst the icons, one photo of the iPhone appears different than the others. It has been zoomed in and set alongside the usual iPhone icon for comparison. Direct size comparisons can't be made, since the smaller image was simply zoomed in to the approximate size of the larger image. There are, however, some clear differences to the outer casing.
The significance of this is unknown. While it could represent an early peek at the subtle design changes of the next iPhone, it may simply be an old "artist's rendition" that was never replaced.