Every time I start up I first use a 800x600 resolution, then I use XrandR to resize back up to 1680x1050. To Workspace Switcher applet does not recognize this and only shows the top-left corner. For example if I have a window in the center of the screen, the "preview" icon would show a window taking up the bottom-right corner of "icon".
*** This bug has been marked as a duplicate of bug 7511 ***