I have just been passed this information by a friend whos dog is currently fighting for his life because of this.
Xylitol, which is a sugar substitute found in many things including gum and toothpaste can cause liver failure in dogs. Brack (the dog mentioned above) ate some without his owner knowing, and it was only because of an ace vet that the cause was even known. There are no warnings on packets which Brack's owner is almost definately going to campaign for.
The scientificky stuff is outlined below:
www.cababstractsplus.org/veterinarysciencedemo/articles.asp?ArticleID=16106&action=display&openMenu=relatedItems&SubjectID=110
(P.S, Feel free to cross post this everywhere you can think of)