I've tried using the 125% and 150% DPI modes in Windows 7 on a 1080p display and the support is still rather primitive. My HP ENVY 15 actually shipped with 125% mode enabled by default. Lots of icons, graphical elements, and many fonts become blurry/pixelated when enabled.

I'm guessing a reason we haven't seen high DPI screens from Dell/HP/Sony/etc is partially because the Windows OS doesn't really support it that well just yet. Hopefully things have changed in Windows 8.