I get it quite often and even after a reboot it reappears. I have a totally maxed out i7 iMac. Probably something to do with crappy ATI drivers. Also happens in column view, and I have also noticed scrolling in either view on my trackpad is absolutely terrible, it stutters and sometimes doesnt scroll at all, then the artifacts appear like in your image.
it has nothing to do with reboot, because I shut down and turn on computer every day and it is there always. Few times scroll in list view and suddenly this appears.