I’ve edited this post since I originally posted it as someone pointed out that it wasn’t just the Pi controlling the printer. Doh!
Daniel Blair has hooked up his Pi and an Arduino to the printer, a Printrbot, and used a piece of software called OctoPrint on the Pi to drive the printing. There’s even a Pi distro you can download called OctoPi if you wanted to do the same, but make easier on yourself. You can view a video of Daniel’s printer in action below or read more on his blog post.