Here is some info I found on acorns.
"The primary toxins found in the plant material are tannins and phenols. These compounds attack proteins they come in contact with possibly causing ulcers and bleeding of the mouth lining, the esophagus, and the intestines. This may be demonstrated by bloody or dark diarrhea. Additionally, the toxins are converted in the digestive tract where they may enter the blood stream causing hepatic (liver) and renal (kidney) damage and in extreme cases failure.
The symptoms are usually delayed, in some cases a week after ingestion. These signs may include depression, constipation or diarrhea (both accompanied by blood), abdominal pain, rough coat, a period of frequent urination followed by no urination.
The lethal dose for dogs is not known, for cattle it is approximately 6% of the animal's body weight. If you have a 40 pound dog, that's almost 2½ pounds of acorns. However the literature does not state how long a period of time this quantity could be consumed over and still be lethal. As a precaution, keep all areas available to your dog free of acorns and downed oak vegetation