Nobody mentions Zachary Taylor, though he did die in office. "Old General Taylor died long ago, walk him along John Carry him along. Old general Taylor died long ago, carry him to his burrying ground." Or so says the shanty. A number of presidents survived assasination attempts; Andrew Jackson, Theodore Roosevelt, and Harry Truman among them. All of which puts a dent in the zero theory.
This having been said, if Bush were to buy the farm before November '04, I would have a hard time feeling sad.
Is there an attractive young woman out there, willing to perform a sexual act on him, and then tell Ken Starr? That seems to be the best way to get presidents out of office these days. (I am neither young nor attractive, or I would make the sacrifice.)