Minecraft.Print() is a Python script that will convert any in-game object into a printable object. By placing identifier blocks at the corners of the object you can select a certain part of the the environment and then run the script to output it to a 3D printer.
MIT Media Lab researhers Cody Sumter and Jason Boggess built the system and you can see recent prints in their gallery.
The script doesn’t appear to be available for download but heres hoping we can soon all start printing working models of the Starship Enterprise.