The PriNXT may not be the first functioning printer made of Lego, but given that it was made by 14-year old Lego Mindstorms wunderkind Leon Overweel, we won't hold that against it.
The printer employs three motors to control the motion of a felt tip pen. Overweel converts source image files into to a text file of 1s and 0s using Paint.NET, which is then (after some tinkering) imported into RobotC - a development environment used to control Lego Mindstorms NXT robots (among other things) based on the C programming language.
Two of the PriNXT's motors control the pen along the X and Y axes, while the third lifts and lowers it from the page. The printer also makes use of two touch sensors, a light sensor and a color sensor. The light sensor is cleverly used to assist with the vertical motion of the pen.
This is by no means Overweel's first foray into Lego engineering. Only last month, he blogged about his Skype-controlled NXT Car. Check out Overweel's video of the printer fulfilling its raison d'être below.