Hmm, various rumors floating around about the next iPhone.
If it really does get a 4" screen, especially as rumors have said it's wider, maybe Apple will increase the resolution. If my math is right you can do about a 3.8" screen at 640x960 and have it be above the 300dpi Retina number.
The picture shows black "wasted" space for un-updated apps in the bigger screen. If the black goes closer to the physical edge of the iPhone, it's only a little more space than the bezel around the current iPhone 4. And might even be less than in the 3GS if you count to the outside of the curved edges.
In the short term, it's a bit of a pain for programmers. But in the long term, and these are platforms that will be around for many more years or decades, iPhone and iPad then have the same base resolution going forward. That would be a huge deal. One set of graphics to cover both devices.