Working on one of my projects I wanted to add a torn look to the edges of fabric. After some experimenting I finally figured out how to do it in Zbrush, and it looks pretty good. Quick overview of it is your using a weave pattern with Micro Mesh to Geo to get you torn fabric, but doing the whole model this way will create an insane amount of polys, so figured out how to just make the edges geo and the rest of the model using Micro Mesh during the BPR. I go into detail below. Like to see what you guys can produce with this effect, so feel free to post your results.
Also supplying you with my weave pattern ztool to get you started.
(Update: Click Align Edge on the Modify Topology next to Micro Mesh Button to prevent the Micro Mesh from flipping or rotating your Micro Mesh.)