That is not a thing!
it's either 2880x1800, or it is 1440x900.
The OS has a "doubled screen elements" thing that functions basically the exact same way that Windows XP's "use big text" accessibility option worked...
If Blizzard releases a "retina-aware" option for D3, it will just double the size of the IU elements on a 2880x1800 screen, while the 3D portions of the game will continue to be 2880x1800. this would make the IU take up more screen room at the expense of gameplay visibility. There might be a happy medium in between that would make sense when you see how small some of the UI elements will be at 2880x1800, but that won't make the game run any faster, because the 3D portion will still be rendering 2880x1800. "retina" doesn't mean anything. You can turn on "retina" mode on any Mac with 10.7, and all of the screen elements double in size. It's a good option to have, but believe it or not, some people might want to have non-doubled 2880x1800 available all the time on their macbooks. That would allow for some significant reduction in the screen real-estate taken up by UI elements and toolbars/panels in various production apps.
A game can't "run" at 1440x900 AND be "retina" because by definition, 1440x900 only has 1440x900 pixels. Unless you mean that the UI is set up to look like 720x450.
Yeah, I should have emphasized that it only retains to the UI elements. However Blizzard can still make optimizations and render at a higher resolution and then scale to 2880 x 1800 like certain iPad apps does.