A recent Make blog post, titled "Proscribed Printables" details how someone designed and printed a functional magazine for an AR-15 rifle.
This is a perfect example of what I've been saying all along - that the idea of "things" is changing. Give it another 5-15 years, as 3D printers that print in plastic become cheap household items (as laser printers have), and then as 3D printers that sculpt in metal become more accessible and mature, and it will be like having a Star Trek replicator in your house.
I think it will fundamentally change the way we think about a lot of products, and how much they cost. The current cost of goods is often determined most highly by the cost of the labor, and intellectual property ("idea") development, that went into its creation. Giving people the ability to use their own relatively-unskilled time and labor manufacture items at home, using things like 3D printing or CNC technology, could have a drastic effect on pricing and business models.
We are seeing a similar trend today, because of internet-based stores with super-cheap pricing, as big name iconic "brick and mortar" stores, such as KMart, Borders books, Blockbuster video, and Ultimate Electronics, struggle to compete with their online competitors. Amazon recently announced that it now sells more e-books, for electronic readers such as the Kindle, than it does tangible paper copies. How will consumers react when they can buy items, such as a plastic kitchen serving spoon or a cell phone case, for pennies by purchasing a 3D model design download, and then printing it at home in an acceptably short amount of time? It certainly seems to change the fundamental concept of what constitutes a "product", and the way those products are sold.
At-home 3D printing also brings the notion of a "designer" into the information age, with all its pro's and con's. On the positive side, the ability to make a worthy design will no longer be left only to the highly-funded companies that are capable of producing the product for profit. However, it also means that products that should be designed by educated professionals because of safety, such as car components that must be designed to meet a certain strength, could be created by any kid with a 3D modeling program and then downloaded and brought into the physical world by someone naive enough to not know the difference. Never before were either of these situations possible (or at least very realistic).
The article about the rifle magazine also hints on some of the legal aspects that could affect people with 3D printers. For a several year span, there was a federal ban on the manufacture of firearm magazines with a capacity of more than 10 cartridges. The ban has since expired, but the typical penalties for firearms misdeeds are very stiff. While the actual 3D-printed magazine design only holds an always-legal 10 rounds, it could be modified in seconds, with minimal skill, to hold as many cartridges as you want. If the 10-round ban was still in effect, a 10 year old kid could face dozens of years in prison, and hundreds of thousands of dollars in fines, for "manufacturing" an illegal firearm component. Just as anyone who has accidentally browsed across a pornographic picture while at work knows, combined with every increasing laws and regulations, it has become extremely easy to break very serious laws these days.
At-home 3D printing and CNC, "additive" and "subtractive" manufacturing respectively, are going to have a profound impact on our lives in the years to come. It will make products easier to get, empower new generations of unshackled designers, alter the pricing models for goods and services, and change the way we interact with laws. I can't wait!