Chocolate contains theobromine. A naturally occurring stimulant found in the cocoa bean, theobromine increases urination and affects the central nervous system as well as heart muscle. While amounts vary by type of chocolate, it’s the theobromine that is poisonous to dogs.
From: http://www.petcaretips.net/chocolate-poisoning-dog.html
Very interesting! My dog accidentally got hold of three boxes of bakers chocolate (she survived, somehow). We think the cat knocked it off of an upper shelf. At that time, my vet said it was caffeine overdose, but I was too frazzled to discuss it with him. Your info in interesting.