The current Retina MacBook Pro has a resolution of 2880x1800 and a diagonal dimension of 15.4”. That gives it a pixel density of approximately 220.5 PPI and a 16:10 aspect ratio.
If Apple were to make an rMBP with a 13.3” diagonal and a 16:10 (the same size as the existing MBP), 2880x1800 would give a PPI of 255.5, 2560x1600 would give a PPI of approx 227, and 2304x1420 would yield a PPI of 204.28. Using a panel with any of these resolutions would mean that it would require a whole different production line than either the existing rMBP or the iPad3.
This gives Apple three options if they want to leverage existing production lines: change the diagonal size, change the resolution to something non-standard, or change the aspect ratio.
Changing the diagonal size would mean abandoning the 13.3” standard diagonal size that they’ve been using for a while now, and would require a 13.7” diagonal at a resolution of 2560x1600 to achieve the same PPI. I think this is fairly unlikely for Apple to do.
Changing the resolution to non-standard is simply not going to happen. They could, theoretically, taking advantage of the non-integer scaling that they are doing on the rMBP at higher/lower resolutions, but I don’t think this makes a lot of sense for them to do as it would incur a performance penalty.
The most likely option would be to change the aspect ratio. Switching to 16:9 instead would allow them to use a 13.3” 2560x1440 and get a PPI of 220.8. When you figure in the inaccuracy of the diagonal measurements and it is entirely possible to make a 2560x1440 panel using the same process used on the existing rMBP panel and still be able to call it 13.3”.
I originally thought that having a different aspect ratio for different sizes was a very un-Apple move, but the current MacBook Air does exactly that - the 11” model uses a 16:9 aspect ratio and the 13” model uses the 16:10 aspect ratio found in the rest of the MacBook line.
I don’t know if that is what is going to happen, but I think it’s at least a possibility.