This was done using the following setup:
50mm f1.8 II reverse mounted to a 75-300mm f3.5-4.5 (using a 52mm to 55mm filter coupler) which was then mated to my 5D mk II.
So in essence, I was using a 300mm lens as a ghetto extension tube with a poor-man's macro lens (50mm lens reversed). The images are pretty poor quality and are quite hard to focus/capture. But here is an example of what could be done with this setup:
Close-up of the number 100 printed at 10pt on plain paper.
This is a cluster of modified leaves (thorns) on a potted cactus I have. Notice the very shallow DOF - I set the 300mm lens to f45 and it still produced a weak DOF. Focus stacking would solve this, however even getting the subject into focus was an epic battle due to the sensitivity of this setup. For example, roughly 0.1mm of movement threw the entire scene out of focus.