I've used Google Sketchup in the past to design robot parts and sometimes to create 3D graphics to illustrate articles and manuals. It's extremly handy, easy to use, and the price is right. From time to time I run across other robot builders using it, but I'm not aware of any forum, blog, or website specifically devoted to robot design using Sketchup.
A good example would be the 'Design. Click. Build' Blog that features all types of tips, tricks, and techniques for applying Sketchup for woodworking. If anyone knows, or publishes, a similar website for robotics, please let me know.