Rhino does draw at 2880x1800 on the retina laptop. All those pixels visually take as much space as a “normal” 1440 x 900 display.
This is most obvious if you plug in one of Apple’s 27" displays to your laptop. The external display is 109 pixels per inch; the laptop display is 220 pixels per inch. That’s almost a perfect 4 to 1 ratio.
There are two ways to handle this difference, and OS X and MSWindows take opposite tacks.
On OS X, drag a Rhino window from one display to the other. The apparent size of the window does not change, but it is much sharper and crisper when drawn on the retina display. That’s because OS X and Rhino are drawing 4 times as many pixels to fill the same amount of physical screen space. Four pixels on the retina display are the same physical size as a single pixel on the external display.
When Apple introduced the retina laptops, it did a bunch of work in OS X to make this work almost seamlessly. Text and standard controls scale themselves automatically. Applications that do custom drawing and do not know they are running on retina displays continue to work; they just look fuzzy on a retina display. So OpenGL drawing still works, but applications needed to make adjustments get OpenGL drawing to look sharp. Rhino pays attention, and you get very crisp viewport drawing on a retina display.
MSWindows is different. There, a pixel is a pixel, and Windows does not seamlessly scale text and drawing to accommodate different pixel densities. Dragging a Rhino window from an external display onto the retina laptop display shrinks the window quite a bit, and makes it hard to read. This is getting better, but it is a bit of a piecemeal approach to fix this on Windows.