New Scientist has an article about how a robot builder could 'print' houses. This is an extreme rapid prototyping machine that prints walls by spraying layers of concrete (or other semi-liquid material) and comes back with little trials to smooth out the walls it just printed. This rapid protyping (RP) machine can build domes, and walls in any shape. Would you want your next house to be printed to your specifications? The robot hangs from a large gantry over the spot where you want your house built and it makes a multitude of passes like a huge printhead printing walls from the ground up. In contrast, smaller 3d printers use a liquid polymer and a laser to solidify a layer and then drops down for the next layer to be put on top. Other 3d printers squirt powder and hardener or blobs that self-harden to create a 3d object upward. Some RP Machines can even create simple electronic circuits and moveab le parts and even printing medicine caplets. I suppose you could even hack your own rapid prototyping machine out of a computer and a dremmel tool. Cheap 3d scanners are nearing the desktop so it may be possible to scult a small house and then have a robot print it in large size someday! Star Wars Permacrete robot building may not be far off!