Deckard is indeed a replicant. A number of clues, such as the one that has Gaff leaving a unicorn origami figure in Deckard's apartment are proof enough. However, the director admits that Deckard was not meant to be a replicant according to the initial storyline.
I think that is part of the puzzle -- viewers have to decide for themselves whether or not Deckard is a replicant. The first time I watched the movie, I didn't see that. But the second time I saw this as a clear possibility.