I think the majority of people are missing the point of the Retina display. Apple is intending, and has designed, the Retina resolution capability to increase pixel density, as in give a same sizes object on another 15" screen more detail. The 'optimized' Diablo 3 experience will not be 2800X1800. it will be 1400x900 with double-density detail.
If you go to your current monitor and increase/decrease the res, you will see the less/more real estate factor come into play. If you took a 15" screen and actually moved the slider to 2800x1800, you wouldn't be able to read anything or see any detail. It would simply be too small.
The perfect example, that I'm sure at least 4/5 of you have seen, is the comparison of the iPad 2 v. the New iPad. The New iPad doesn't have twice the real estate. The apps simply use a higher density pixel count at the native iPad resolution. MAJOR DIFFERENCE. An old app doesn't display at half the size, just half the detail.
I'm sure you could only get 20 fps on 2800x1800 on Diablo 3, unless it was an OPTIMIZED, HIGHER DENSITY version that Blizzard will prep specifically for the Retina Macbook. Until then, we will see people complaining about slow fps, unsuitability, etc, etc.
UPDATE- I installed DIII on my new Macbook Pro Retina last night, with 16gb ram as only upgrade. I disabled auto graphics switching, so it is in full performance at all times. At 2880x1800 on full screen mode, all settings at high, with normal (mid) shadows, the performance is silky-smooth, and no slowdowns even on act 2 inferno with my monk. The colors are amazingly vibrant, the action crisp, and I have not seen any hiccups thus far. DIII never looked so amazing.
Edited by Titanigon#1913 on 6/26/2012 10:55 AM PDT