James Sesler posted this video featuring Arduino controlled pen plotter attachment for your 3d printer. I hope he release more information soon and makes the plotter design and code publicly available. Kudos James!
There are many pen plotter attachment projects on Thingiverse:
http://www.thingiverse.com/search/page:1?q=pen+plotter&sa=
This will probably be one of my future projects. Can you guys recommend some more resources on how to make everything work base don real experience?