OS X uses sub-pixel rendering for anti-aliasing at the "for Flat Panel" setting. When using a rotated screen (supported under OS X via ATI X1900 XT drivers), the sub-pixeling is broken.
http://discussions.apple.com/thread.jspa?threadID=652279
Have a look.
blakespot
http://discussions.apple.com/thread.jspa?threadID=652279
Have a look.
blakespot