It is an interesting resource open to designers, and there are companies looking to roll out 'fabrication' centres in various places we already go for printing services. Saw something a few weeks about about Staples or another business supply chain apparently planning to start adding such services to their stores in the next while. Things are only getting faster, more reliable, and cheaper.
Something that looks really promising are multi-head print'n'mill setups for composite materials. One of my friends has a neat one setup that combines an automated stock milling machine with a set of robotic arms UV cure epoxy heads, and wrap spoolers for fibreglass or carbon fibre. When you set up a 'print job', it will begin by milling aluminium stock parts (if used in the model), set them aside, and start constructing stuff out of thin layers of fast UV cure epoxy and fibre strains.
The real neat trick is that is uses multiple cutting heads as well as printer heads along with the fibre spooling. It will print and mill in one setup, so rapidly lays down material in a semi-rough fashion, and then come back and do fine finish details by removing excess material and polishing. The coolest part is that not only can the machine mill its own parts, but the arms can actually be turned so that it will assemble the copy of itself (And will bring the new robotic arms online to help finish... Originally had the problem of not actually being able to reach the far side and needed someone to turn the piece around.), and after building your second copy, they can be linked to share parts fabrication between each other and pass stuff back and forth. (Plus they can daisy chain!)
Something the size of a human hand can be done in about 30 seconds if voids are allowed.