Woah!!! I created a inefficient but working 3D printer in Minecraft. The 3D printer is built with the ComputerCraft mod. The turtles within the mod are blocks that looks like small or big computers depending one you Minecraft perspective. The turtles are controlled by LUA scripts with a comprehensive API to control the turtle.
The 3D printing script is hosted on pastebin for your convinience, http://pastebin.com/7Uukwe9b
To install it to your turtle, type pastebin get 7Uukwe9b printer
The first editor for creating instructions to the 3D printer is located online here, http://hosting.olavz.com/minecraft/editorr/
It its currently limited to a 2D view and by adding layers, you are working your way upwards along the Y axis. Remember to add layer before you try to export. Copy the exported data and create a new paste on pastebin.com. After you submitted the data to pastebin, look at the last ending characters in the url. Example: http://pastebin.com/5uzbb6P0
Going back to the turtle you install the 3D printer as printer.
Type printer <pastebin code>
Example: printer 5uzbb6P0
Slot 1: Fuel
Slot 2: Resource
Slot 5: Ender chest with fuel
Slot 6: Ender chest with resources
In addition to the 2D view editor. Morgan Sandbæk is working on a 3D editor with import/export capabilities so the two editors can be used in a combined effort. The 3D editor is still in development, however I will leave you with a teaser.