Interesting you should ask this question as I'm working on exactly the same type of thing:

1. see the attached image
2. also check this image out http://www.renderosity.com/mod/gallery/index.php?image_id=1838272

It's a bit tricky when using displacement on curved or irregularly shaped objects/terrains.  I was able to come up with something by using World Cylindrical mapping with a Squares noise node.  I'm still working to try and perfect this and then I will Metanode the variables (e.g. roughness, spacing, etc) and then throw in into my Rock My World - Rock Generator.