@steveclayton said:
For Mac
Download GraphicConverter: http://www.lemkesoft.de/en/products/graphicconverter/download/
Select: Download now (from Switch.ch)
Once you open the Free version of GraphicConverter, File/Open and find the folder, where you have placed all your Sketchup files. Under EDIT / Select All
Now at the bottom click on (Create Preview) This will change all
the Sketchup logo(s) into it's graphic thumbnail.
Note: It will not launch the graphic icon, unless you are in Sketchup.
Open Sketchup and (Open File). You should see the Thumbnails, and these icons will launch. 
Steve, I could kiss you! I've been wanting to be able to see thumbnails of my SU files since I got my mac 4 years ago. Thank you so much for this tip! 