It seems to only do this when I have Safari in fullscreen. I'm running a 2011 baseline iMac with Lion 7.1. When I switch to a different screen the top toolbar remains, so I have to exit fullscreen mode in Safari to make it stop. I included a screenshot of it in action. Notice how the top of the screen has the Safari toolbar stuck. I don't know if that portion is called a toolbar but I'll use it till someone corrects me.