Just played around in PS with this... Created an action for it too.... Its pretty much the same steps as Rob said...
>copy, create new, paste, increase image size.
>Gaussin Blur 2.0 pix
>New layer set to color burn
>Filter>Render>Clouds. Filter>Render>Difference Clouds.
>Threshholds
>Lather>Rinse>Repeat