I just finished a an OSL shader that creates patterns that resemble fingerprints.
(yeah I know the is some darkening around the print itself in the example image but using more samples and especially more transparent bounces solves that)
As usual the OSL shader and an example noodle are given on my blog:
http://blenderthings.blogspot.com/2012/12/a-fingerprint-osl-shader-for-blender.html
In fact, the noodle is rather complicated and uses two different OSL shaders: one of very general utility that creates an elliptical mask and very specific one that creates vector flow fields. The latter one can be used to create the banded patterns with knots that you see in fingerprints. How this is put together is all in the article.
Let me know what you think of it an I am especially interested in any links to other people who might have written something about using flow fields in shaders to generate natural looking banded patterns.
cheers