Seriously: This is going to be nerdy, pointless, and probably offensive to some other geeks with deeply-held opinions about science-fiction movies. Depending on your feelings about "Blade Runner", you might want to turn back now.
OK, going to assume anyone who's still here either agrees with me, or at least won't take it personally when I say the following: Deckard is not, I repeat not a replicant. Because if he was a replicant, then "Blade Runner" would be entirely pointless and uninteresting instead of just being mostly pointless and uninteresting.
For one thing, "Deckard is a replicant" is the laziest, hackiest, dullest possible ending anyone could come up with given the basic set-up of the film. Seriously, it's the goddamned Tomato in the Mirror ending! "Dude...what if the guy who was hunting replicants...was a replicant himself? Whoa. I think I just sprained my brain." It's the sort of thing that you think of in the short story you wrote in eighth grade, not the ending you want to give your multi-million dollar budget sci-fi motion picture.
Also, it doesn't make a ton of sense. Yes, Rachel's example does show that you can make a replicant with false memories, one that believes itself to be human, but what about everyone Deckard interacts with? Are they all in on it, every single person in the movie, and pretending to have known Deckard for ages in order to provide verisimilitude for his memory implant so that he won't go rogue? (Except for his partner, who apparently knows he's a replicant but only tells him by leaving cryptic hints in the form of origami unicorns.) And would they really let a replicant wander around without his partner as much as he does, given that they're supposed to be so highly dangerous? (Speaking of, if you were going to make a replicant to hunt replicants, wouldn't you make one that isn't quite so likely to get his ass kicked on an almost-continual basis? If Deckard's a replicant, he's a candy-ass replicant.) And would they really, at the end, not be waiting at his apartment in force to retire him if he was actually a replicant? That's a lot of plot holes that get resolved simply by taking things at face value. Occam's Razor alone tells us Deckard was human.
But more importantly, it misses the whole point of the film, which is that all of the stated reasons society gives for needing to hunt down the replicants are entirely false, and the real reason that Blade Runners hunt down replicants was the same reason plantation owners hunted escaped slaves. Their entire society relies on replicants doing the dirty, nasty, ugly jobs out in space. It relies on disposable human labor. So they have to convince themselves that "replicants aren't like us." They're not humans. They're things. You can do whatever you want to things.
But what's the supposed difference between "replicants" and "people"? Replicants lack empathy. They don't care about anything but themselves. They can't feel for others. Simple, and apparently even testable. (Although gosh, you have to wonder how many entirely human sociopaths failed that test and died unmourned.) The people have feelings for each other, the replicants don't. It's how you know who to kill.
But that's what they tell each other in the movie. What do we see? We see replicants banding together against a strange, hostile world. We see them mourning each other, avenging each other's deaths...and most importantly, we see the last act of Roy Batty. He saves Deckard's life at the end, an act that has absolutely no explanation other than as an act of pure empathic kindness. He gains nothing from it: He's in the last minutes of his life, and saving Deckard or letting him die won't prolong it. He has no reason to care about Deckard--the man killed his lover just minutes ago. He saves him because he looks down, he sees Deckard struggling pathetically like an overturned turtle, and he feels something.
And on the flip side, we have Deckard. A man who's such a good Blade Runner that his own partner can't help but wonder if he's a replicant under some sort of deep cover, simply because Deckard has spent so many years gunning down things that look like people that he's had to shut down all his emotions just to keep from committing suicide. Deckard quit because he found out he was forgetting how to be a human being. How many Blade Runners could pass the Voight-Kampf test? How many of them have lost their empathy just to survive a job that involves killing people all day, every day?
That's the point of Blade Runner. It's about a replicant who acts like a man, saving a man who acts like a replicant. It's about the lie at the heart of the world Deckard lives in. Once Deckard has seen that lie first-hand, once he's learned that replicants can love others selflessly and truly, he can be with Rachel at last. He can start remembering how to be human again. (It's no coincidence that the Deckard in the book had a wife that was written out for the movie.)
It's still not a perfect movie. It's so bloodlessly subtextual that there's almost no text to it. It's a movie about slavery where nobody ever talks about slavery, a movie about a man who realizes that it's just as wrong to murder artificial humans as natural ones, but who never gets around to saying so. It's a movie that actually did need a voice-over, just not the one that we got. We needed to be inside Deckard's head; instead, we got something added in post-production by nervous execs.
But if it were really about Deckard being a replicant who doesn't know it? Then it would be utterly without redeeming value as a story. It'd be nothing but a bunch of pretty aesthetics, a decent performance from Rutger Hauer, and a Vangelis score that plays about three times louder than the dialogue. And the worst part is, sometimes I think that's what most Blade Runner fans actually want.