Retina screen workings
The OS is actually rendering at 2880x1800, while "thinking" it's 1440x900 (Similar to what the retina display does on the iPhone). So when displaying on the screen, everything looks very sharp and crisp, since we have 4 times the level of detail of a regular screen (2 times in each direction). And by "thinking", I mean that the OS uses the high quality graphics and fonts whenever available. So for example if the Safari Icon is normally 64x64, it will use the 128x128 version instead. And it is the case for every icons on the screen. If you dig around the OS a little bit, you'll see things like iconname.png and firstname.lastname@example.org
The retina "magic" also happens at 1680x1050 and 1920x1200.
on the 1680x1050 case, The OS is rendered at 3360x2100 in the buffer "thinking" it's 1680x1050. So it will use the same high res icons as before. Then, using fancy Math algorithms, the 3360x2100 is scaled down to 2880x1800 to fit the native resolution of the screen. That algorithm basically finds out what pixels to keep and which ones to combine.
That is why it is less clear than the native retina, but still better than non retina screen. You will also notice that Apple gives a little warning saying that scaling resolution may affect performances. That is because the GPU is rendering at that high 3360x2100 resolution internally, which is a lot of pixels, then the scaling algorithm on every single frame also affects the performances.
1920x1200 will be rendered in a similar fashion at 3840x2400 then scaled down to 2880x1800.
However, on Linux, I don't think there is support for HiDPI (the "magic"). If there is, I haven't figured it out yet. So, the internals of the system don't have high resolution icons stored with the regular icons. so the only thing we can do is render at resolution lower or equal to the native resolution 2880x1800, using the regular icons and then scale up. That's why everything will be readable and bigger but blurrier. If you were to run at resolution higher than 2880x1800 like what OS X does, it would still not make a difference because Linux will still be using it's low res icons and scaling down will just bring you back to normal and that would also be a waste of battery and GPU. Linux will also give you a resolution out of range error because it doesn't support HiDPI yet.
Note that system fonts are a little different since they are based on SVG (as opposed to png) and can be manipulated independently of the screen resolution. SVG stands for 'scalable vector graphics', which means you can scale fonts as you wish and they will just automatically scale up and down and look awesome!