Just noticed this happening in SL the last few days - app icons in my dock are starting to become invisible. The white dot underneath shows that it is running, but there is no icon. The icons are present on the apps themselves, but also do not show up when I look at stacks. Any clue as to how to fix this, or is it just yet another SL bug? Just to add: killing the Dock doesn't fix it, quitting the Finder doesn't fix it, and restarting the computer doesn't fix it.