"King's Crown" said in
rec.pets.dogs.health:

The chemical in chocolate that is
supposed to be bad for dogs is one that is similar to
caffeine. I can't remember the name.

Theobromine.

Anyhow the dog needs
to eat quite a lot of it to really make them "take to the
vet" sick.

How much chocolate that a dog requires to make it sick depends
upon so many factors that I wouldn't be comfortable giving the
above advice.

If your 8 pound (3.6 kg) dog ate 8 ounces of milk chocolate,
he may have ingested as little as 350mg of theobromine. The
toxic dose of theobromine can be as high as 200mg per kg of
dog weight - in your dog's case, 720mg could be the lethal
dose.

IOW, your dog may have eaten half the lethal dose.

At the other end of the spectrum would be a more sensitive dog
eating better quality chocolate. Your dog again: he eats 8
ounces of baker's chocolate, ingesting as much as 3600mg of
theobromine. The lower end of the toxicity threshold is about
100mg per kilogram of dog weight, or 360mg.

Your dog would be dead 10 times over.

--
--Matt. Rocky's a Dog.