I'm a believer. 3D printing is going to change everything from how we create to how we consume. Once they become more affordable, you're going to see people printing everything from new toys to new livers. Of course, 3D printers have so far only been seen creating small-scale projects, but what about large-scale projects like housing? It can do that too.
The first technology to emerge on the scene was a technology called Contour Crafting. It was invented by Behrokh Khoshnevis, a professor of Industrial & Systems Engineering at the University of Southern California. His system is essentially a giant 3D printer can construct a home in less than 24 hours. This isn't just laying the foundations and building walls either. Contour Crafting would be able to lay down the plumbing as well.
Here's a video of Khoshnevis talking about Contour Crafting and how it can aid in construction of cheap, yet sturdy, housing in poor nations and low-income neighborhoods.
Another school in the UK has also come up with their own take on cheap construction - 3D Concrete Printing. It's similar to Contour Printing except that it's main purpose is to print the concrete in unique designs to complement the construction of a building. It still promises to make construction easier, cheaper and safer.
Like most 3D printing projects, this technology is still primarily in the realm of scientists and researchers. The exciting part is that this technology is getting cheaper and cheaper all the time. It's only a matter of time before people will begin downloading the design to a house and then printing it off themselves. I, for one, can't wait until we start downloading houses.