From Ars Technica come a report of the first 3D printed gun. It isn't technically completely 3D printed, as they say in the article:
The Liberator pistol isn't all 3D printed ABS plastic, though. There are two bits of metal within the design: the first is a nail, which serves as the firing pin (producing a firing pin out of plastic is a difficult proposition). The second is a 6-ounce steel slug, used to ensure the weapon can be picked up by metal detectors as required by the Undetectable Firearms Act of 1998.
We've entered a new age in the discussion over gun control with this. I can just head on over to one of the many web sites that sells 3D printers, purchase one and then print out an AR15 equivalent. How to view this in terms of legal issues seem easy although the practical issues of enforcement are likely to overwhelm any attempts. Obviously, current laws would cover the printing of these guns as manufacturing firearms and regulate it as such, but on a practical level there isn't a whole lot stopping me from going out and dropping a thousand dollars on the supplies needed and then just printing up an AR15. It seems at this point that getting guns off the street just got that much harder.