I just looked around for more on this, and apparently it's a pretty long-debated discussion on whether he is a replicant or not. I haven't read Do Androids Dream of Electric Sleep, the book on which Blade Runner is (loosely) based, but apparently in the book Deckard is unequivocally human. Ridley Scott (who also did not read the book, supposedly) has since said that Deckard is a replicant, but there are no visible clues in the movie to indicate it either way. Harrison Ford has said that he believed Deckard to be human.

So, the book's author and the actor playing the character believe he is human, and the film's director believes he is a replicant. We can take that for what it's worth. Perhaps Scott was hoping that little mystery would keep people talking about the film, effectively lengthening its life. Do you think the movie would be as acclaimed today had the answer been explicitly stated outright?