For Fedora 17 Beta RC3 the Xorg maintainers decided to switch Cirrus driver to 16 bpp depth by default for virt. evironments (as the Cirrus/VNC combo in KVM) due to the huge breakage of Gnome Shell. The reason is that 24 bpp (32 bpp is not supported on Cirrus) code path is not very well written/tested/used (but for Qt has been already fixed...).

You can see the image quality on attached screenshots (KDE Plasma Workspaces but I expect I can write a simple reproducer using QPixmap) - the raster one is corrupted, the native one is ok.

See https://bugzilla.redhat.com/show_bug.cgi?id=810161 and https://bugzilla.redhat.com/show_bug.cgi?id=809052