not sure if this is a bug, my friend recently bought a mac and is running CS6, the brush preview doesnt seem to be smooth as usual, as you can see the hardness is 50% and should have a soft edged preview display, but thats not occuring, and the brush also draws with hard edges in the preview window, this was the default soft edged brush selected first.
That looks like you might be using the Pencil tool (notice that none of the previews are gray, just black and white).
oh thats great !!. thanks Chris, awesome,