I installed Fusion last night on my MBP with Retina.
In full screen, a XP vm ran fullscreen at 1440x900. It looked great. Although a couple of times when switching into full screen it showed the native 2880x1800 which looked really bad (blurry and small). Toggling out and back fixed it.
I'm new to the whole Mac platform so this may not be news... but when rescaling the fusion app in windows mode, the XP guest would change the resolution to match it perfectly (ie. it would sometimes run in 937x824 resolution).