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As currently specified (http://dvcs.w3.org/hg/webcomponents/rev/769c949cd10c), you can't call cloneNode() on a ShadowRoot instance -- because you can't create a ShadowRoot instance without associating it with a host, and cloning ShadowRoot with associated host will produce nonsensical results. We should see if there's actually a need for a working cloneNode() method for ShadowRoot instances.
Nobody asked for this in a while. Closing.