All things are poison and nothing is without poison; the dosage alone makes it so a thing is not a poison.
—Paracelsus (1538)
And the dosage required to make chocolate a poison is quite a dose indeed! Humans are much better at detoxifying theobromine than dogs; most of the sources I've seen estimate that the median lethal dose in humans is about 1000 mg/kg, compared to a dog's 300 mg/kg. To reach this dose, an 80 kg man would have to consume about 5 kg of unsweetened baking chocolate, 15 kg of semisweet chocolate chips, or 50 kg of milk chocolate.
Symptoms of theobromine toxicity (headaches, trembling, vomiting) would be apparent at much lower doses than these, obviously, but the point remains: most people without risk factors like heart conditions would have to embark on an epic chocolate binge in order to consume a life-threatening amount.
To most animals it is a very toxic food, but this comes in the form of concentration as slime has stated above. Interestingly enough, if you had a pet rat or mouse, it would probably be able to handle more chcocolate than a cat or dog (their metabolism of theobromine is on par or exceeds a human's), but due to their relatively small size, it is best not to give them any.