It's a popular theory that Deckard is a replicant, but it doesn't make much sense. You'd think that a replicant model sent after rogue replicants would have been given mental and physical 'upgrades' to help him, but no, even the pleasure model is able to completely wipe the floor with him. That's a practical reason why he's not a replicant.
Storywise and thematically, part of the story is that the replicants are in many ways more human than the humans. Their desire to live and their group dynamic in that shared goal is a stark contrast to the almost robotic way the humans behave and live, all in isolation in lifeless buildings, apartments or penthouses. Having Deckard be a replicant just undercuts it all. The story would thus become a muddled affair where it's most powerful dramatic angle, that of a guy hunting down his own kind unawares, is missed in favour of one ambigous shot.
In terms of the film's crew, Scott and I believe one of the producers have come out and said that he's replicant, whilst the screenwriter and Ford himself have said that he's not.
"Hah! That's turned you white, hasn't it? That's frightened you, you lily-livered, caramel-kidneyed, custard-coloured cad? Not so brilliant now, are you, eh? eh?"