Ram; Chocolate is more toxic than poisonous. A litle tiny piece given to a fair sized dog should be no problem, but if he eats most of your chocolate bunny when you're not looking, that's not good. I think it also has to do with the amount of cocoa in the chocolate; dark chocolate would be more harmfull than milk chocolate, and high cocoa gourmet chocolate or baker's chocolate worse still. It's just a good rule of thumb to keep ALL chocolate away from dogs. I think it's also toxic to cats, but they can't taste sweet and have no interest in it (and they're too smart for that)!
