(I'm on Mac OS X 10.6.5, though I just upgraded from 10.5 maybe a month ago.)
Since time out of mind, I have been able to copy a screenshot selection to the clipboard (using cmd+ctrl+shift+4), and paste it into a new or existing Photoshop (CS3) file, and then use, say the magic wand to find a contiguous area of color. As of a few days ago, this stopped working so well. The pasted image appears to be dithered. See here:
The region on the left should be solid #FF00FF. In fact, if I do a cmd+shift+4 (which just creates a file, doesn't copy anything to the clipboard), and open that in Photoshop, it works properly. I can also paste from the clipboard into another application such as Acorn and verify that the color is solid. (So, this seems to be a Photoshop issue.)
The only solution I've come to is that if, when pasting into a new document, I choose 16 bit color mode (it defaults to 8 bit), then I don't get the dithering. However, I have never had to do this before. It's always just worked. Is this something that may have changed in Snow Leopard and I'm only just now noticing it? Any other ideas of what may be going wrong?