Good question.
Many people on this board think Michael "signed on" when he visited Vito in the hospital after the bodyguards were pulled. When he said, "I'm with you now, Pop," Michael meant more than just being physically present with Vito--he was through distancing himself from his family and their business.
TB, I believe Michael's words at that moment in the hospital were not a commitment to the family business or anything close to that. Michael was simply stating that his estrangement from his father was over.
As far as Michael committing to the family business after hearing of Sonny's Death, in the novel (after Apollonia is murdered) he tells Tommasino to tell his father that he wishes to be his son. Maybe that's a commitment to the family business.
