Here is an article on it. Yes, it is toxic, and cumulative. It takes a lot to kill a big dog, but the toxicity is measured by body weight, so little guys are more at risk. And my dog, Casper (who used to be my avatar) has eaten at least one very large chocolate bar in the last six months that he stole from the table, so he's well on his way to being a toxic chocolate dog! Now we watch him like a hawk.
Joanie
edit: the articles don't mention a cumulative effect, but our vet did. Erring on the side of caution is always safer.