I've seen the preprocessing and printing process in action at 3DPrint (http://www.3dprint.co.nz
). The preprocessing stage is actually very involved as you need to understand thickness and design properly connected 3D models beforehand. It's a bit like getting business cards printed - you end up paying them to pre-process your files for guaranteed success.
The tool prototypes in the video are ideal subjects as they can be designed entirely in 3D CAD from the beginning and don't have complex structures joining various parts of the overall model.