The Ben Heck Show had an episode on creating a portable 3D printer. The idea is to fit an entire 3D printer into a suitcase for bringing to fairs and such. The video is a nice guide to the bits and pieces that goes into building a 3D printer – portable or not.
Finally, apparently Ben Heck enjoyed his 3D printer build so much that he created the printbotGo – yet another portable 3D printer. The build (and result) is covered on his blog.