Recently, I was at a Vets office and read an article that grapes and raisins are toxic to dogs.
According to Dr. Gwaltney-Brant at ASPCA Animal Poison and Control Center (http://www.aspca.org/site/PageServer?pagename=pro_apcc_ask_peoplefood&s_state=grapes) grapes and raisins can cause renal failure. It appears it would require more then a couple grapes or raisins for a medium size dog. However you should refrain from letting your dog eat any human food. An update as of February, 2007, says that "as few as 7 grapes can be toxic."
Doing a google search will result in quite a few articles on the subject of grapes and raisins being bad for your dog. In the process of doing this, there are several helpful web sites found.
One such web site is VetInfo 4 Dogs where there is a list of toxins and poisons that affect dogs. Some other toxic things can be sugarless candies, onions and garlic, chocolate and more. Please see the article or better yet, contact your Vet. Please keep in mind that articles on the Internet, including this one, may not always be accurate so please check with your Vet to make sure on any of these subjects.
At any rate, here is a contact if you suspect your dog has been poisoned and need immediate information:
ASPCA Poison control hotline (888) 4ANI-HELP or (888) 426-4435.
Web site: http://www.aspca.org
Please stay informed to care for your pet.
SuzieQ