There was this show on Discovery Channel last night called Monster Cracker (the guide said it was a special but something tells me it might become a series) and it had a sci-fi expert wanting to "make" a dragon that was feasably possible. When I first heard about the show, I was thrilled. I love dragons, you know that. Anyway, he borrowed things from real animals -- chemical sprays (that he modified into flame-throwing) from bombadier beetles, claws and teeth from komodo dragons, wings from bats, gliding ability from draco (sp) lizards. What he said he couldn't work out was a creature that had four legs and two wings -- he said it would mean one set of shoulder blades on top of another. That just bugged me. You can explain breathing fire, but not six limbs? My idea is a longer torso, so there'd be more room for six shoulder blades and the necessary muscles. I know the two wings, two legs dragons exist in fantasy (I think they're called wyverns) but I prefer the classic Western dragon.
I guess this is why dragons don't exist -- their physiology is nearly impossible.