I have a MacBook Retina running Windows 8 RP, with the driver that comes with Boot Camp.
I understand why running in a none native resolution like 1920x1200 is grainy. But 1440x900 is exactly half resolution of the native 2880x1800 resolution, meaning that there are 4 pixels to paint 1 pixels (aka pixel doubling), and thus should look great.
When running 1280x720 on my 27" Cinema Display (exactly half of 1560x1440) the picture is sharp and crisp, so why is this a problem on the Retina display?
PS: Running 150 DPI is not an option. Windows is really ugly when changing DPI and many applications have UI problems. Many Apps like Chrome and Skype does not support DPI but windows scales them so the look grainy. But worst of all, attaching an external screen forces this to run in 150DPI as well (turning my 27" into an 18"), and some programs like Remote Desktop are note scaled at all, so every thing is very small.