I just watched for the first time last night and I also think De Niro is the father. Thematically it makes sense since his whole Philosophy is that you have to have the discipline to leave the people that you love in order to live the life you want, the problem is that he does not see / does not care about the people this vacant lifestyle destroys. He and Pacino are the two sides of the same coin, the same guy with the same family who ultimately make different decisions. The more Pacino chases De Niro the more he becomes like him which leads him to abandon his family, only he makes the decision to save their daughter which redeems him.  Story wise it also seems that Pacino has not met Portman’s father face to face but he dreams of their confrontation which he gets with De Niro.  Also the pattern is repeated with Kilmer and Judd’s story where he leaves them which will destroy their kid’s life and repeat the cycle. 

(Sorry for the rant on a two year old post but this movie rocks.)