Chocolate is not as toxic to your critters as something like raisins, but keep in mind another VERY toxic item for your pets is sugar free gum, in particular Zylitol which is the artificial sweeter. It can cause massive liver damage in as little as half a pack of gum for a 70-80lb doggie. I've heard some of the little guys only eaten a few sticks of the stuff and was enough to be fatal.

Please keep your gum up and away from your furry family members.\

http://www.snopes.com/critters/crusader/xylitol.asp

Shannon