When you select the equivalent to 1920×1200 option in OS X, what is actually happening, is that it scales the UI 2×, rendering at 3840×2400, and downscales that to 2880×1800.actually I take that back. It works fine!
I just wish retina worked @ any resolution with the new drivers.. sigh.
1920x1200 still looks blurry in bootcamp
In Windows, when you select 1920×1200, you get 1920×1200 at 1×, which is then scaled up to 2880×1800 which is why the image is blurry.
Setting windows to 2880×1800 at the Larger - 150% size should result in an equivalent workspace (2880×1800/1.5 = 1920×1200) though many applications do not properly support itjust like many applications do not support Retina on OS X. (but the way they handle it is different)
Alternatively, what you could try would be to set scaling to be performed on the GPU rather than the display in the Nvidia driver options, create a new custom 3840×2400 resolution, and set Windows scaling to 200%. This is the same method that OS X uses when you select 1920×1200, but I suspect it will produce worse results than Windows running at 2880×1800 and 150% I dont have access to Windows to do a comparison right now though.