You already have access to Shapeway's convenient 3D printers. There are thousands of objects already designed and you can design your own without that much difficulty. Why aren't you using them to make things? Why would it be different if the printer was at Kinko's instead of Shapeways?
The (missing) step between taking the widget to Kinko's and printing the test part is the time-consuming design phase which will be the challenging part for most users. Also that phase is usually lengthy enough that the fab and ship time is not such a significant fraction of the entire process time.
I'm skeptical that 3D printer ownership or ubiquity is the impediment to their widespread use.