I am surprised nobody came up with this at MR(or i missed it).
1152x768
Aspect ratio 3:2
4.0 inch screen size
You could use Ipad apps on that res with very small black bars on each end.
You would still be able to run native iphone apps, with bars on the sides and small bars up and down. (but who would care, I should use the ipad version of that app instead)
This would make it a lot easier for developers with less fragmentation:
-1024x768 for ipad 1/2/mini?
-2048x1536(2x)1024x768) for new ipad
-a slightly taller(1152x768) version for the iphone 5
They also could have gone 1366x768 if they really wanted to go 16:9 and still get the ipad app compatibility, but than the black bars on the sides would be bigger, running an Ipad app.
Apple should have gone 768Pin my opinion. This would be more transparant, and should cause less compatibilty problems in the future.