The Hobbit director Guillermo del Toro paid a visit to the message boards at TheOneRing.net today confirming that he'll be making a cameo appearance as a small monster in The Hobbit:

I had a hand on the design of the creature and I will personally sculpt the appliances that will be applied on my face and hands. I used to sculpt the creatures for NECROPIA (my FX company) and I miss it a bit. I will have a line or two and die quickly.
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Del Toro's comments were in response to a discussion on the boards about a recent German interview he did about about The Strain.