I discussed the Smaug appearance with great detail with [Tolkien illustrators] John Howe and Alan Lee, and there's some illustrations that John did out of that meeting in London that approximate Smaug. I cannot say much more. He is becoming clearer and clearer.
I thought I would post some artwork from Howe and Lee to give you some indication on how the design *might* look. This will be GDT's vision, but there might be some common themes in the artwork that might translate to the design.
What I notice with Howe's art is how the gold and fire illuminates Smaug and gives him a golden color. I wouldn't be surprised to see GDT use that in his design. Also, Smaug has a smaller head in relation to his body - almost like a Stegasaurus. What I wouldn't be surprised to see is the elimination of the "bat wings". Not that Smaug won't fly in GDT's movie - I just know Guillermo likes to think of completely original ideas, and we might expect a style of dragon wing that is completely original.
I think you *could* expect to see Lee's "serpentine" version of Smaug - long and lean and very snakelike. The one problem I see with that is I would expect Smaug to be MASSIVE in the film - and the thin version of Smaug doesn't give off the same sense of power as a fat Smaug. But biologically it makes sense - Smaug could easily navigate the caves of Lonely Mountain if he was thin.
Also, I predict that GDT will go with the "snout" on the dragon face as Lee presents, even though it will be a challenge to get the character to "lip sync" with an actor's voice.
One last note: I think however the design turns out - it will make sense in one way: Smaug's appearance will be indicative of how he lives. So think about how a creature might look if it sits in a cave most of its life, hoarding gold. Would it have small eyes from sitting in the dark? Would it be lean, in order to navigate the cavern tunnels? Would its scales reflect the heat that it breathes to keep it cool? Would it seem wasted and whithered from being unable (and unwilling) to move? These are all things that Guillermo will consider when putting together a design - he will want the design to not only look cool, but he will also want it to TELL A STORY about the creature.