I have a random issue with CS5. Every now and then when painting out on a mask transparency, the thumbnail visual does not show a representation of what was painted black. It is as if there is a delay with CS5 registering the information, but my Mac system is new and bawny. The weird thing also is that I happens on my computer in the studio as well as at home.
The only change I made to my preferences is changing the color of my transparencies to a solid white.