I'll keep saying it, over and over. No larger screens please.
People who attack me for this and say: "Herrderr, why you no like choice?"
Well it's not necessarily choice I dislike, rather fragmentation.
Yeah, because this is such a huge problem on the Mac.
Just use the aspect ratio of the iPhone 4 and scale rasterized graphics in apps that aren't native. At 300+ dpi "retina" resolutions, the scaling would look plenty good enough to be usable, and a lot of content even in non-native apps isn't rasterized and would be full resolution anyway.