The answer appears to be that a coyote (and probably most any animal) can eat a rattlesnake without being poisoned because the venom must enter the bloodstream to be effective. So the animal could be poisoned if it has any internal cuts but otherwise the venom will breakdown in the digestive system. Also, it may be unlikely that a coyote would eat the venom glands of the snake to begin with, even if it managed to kill a rattlesnake. Coyote instincts will most likely tell them that it's not edible. For the same reason they don't eat poisonous toads, natural selection has removed the animals that do.
Assuming all this is true. Can anyone point me to sources verifying (or contradicting) this?