There's also a nice node-input on top of the rock object node called 'mesh displacer', which has a bunch of extra, and hidden, object subdivision options you can use, if you open the rock-object clipfile in a text editor and re-save it.
You don't need a text editor just to use the mesh-displacer's default settings, though, just plug something in. I think you still need to disable/enable the rock node after you plug any new displacement shaders in to make the object wireframe refresh in the 3D preview window...