From "The Straight Dope": "Chocolate is the third most common cause of poisoning in dogs. Certain chemicals in chocolate, notably caffeine and theobromine, can cause erratic heartbeat and in large enough doses can kill your pup."

I know a dog who has eaten chocolate (brownies) and lived, but he was very very sick, and threw up a lot, even after he had rid his digestive system of any of the brownies. His owners got lucky in that they caiught him in the act, before he could finish the whole plate of brownies, which is probably the only reason their pup is around today.