Warning: Possible spoilers galore. You'll really like to experience this film without prior knowledge of the matters discussed here. But you've already seen it a hundred times, right?

The "FOR" case

• Ridley Scott and Harrison Ford have stated that Deckard was meant to be a replicant. In Details magazine (US) October 1992 Ford says:

  "Blade Runner was not one of my favorite films. I tangled with Ridley. The biggest problem was that at the end, he wanted the audience to find out that Deckard was a replicant. I fought that because I felt the audience needed somebody to cheer for."

• The shooting script had a voice-over where Deckard says, "I knew it on the roof that night. We were brothers, Roy Batty and I!"
Gaff knew that Deckard dreamt of a unicorn, therefore Gaff knew what dreams that Deckard had been implanted with. (BRDC only)
• Replicants have a penchant for photographs, because it gives them a tie to their non-existent past. Deckard's flat is packed with photos, and none of them are recent or in color. Despite her memories, Rachel needed a photo as an emotional cushion. Likewise, Deckard would need photos, despite his memory implants. Rachel plays the piano, and Deckard has a piano in his flat.
• Gaff tells him "You've done a man's job, sir!". Early drafts of the script have him then add: "But are you sure you are man? It's hard to be sure who's who around here."
• Only a replicant could survive the beatings that Deckard takes, and then struggle up the side of a building with two dislocated fingers.
• Bryant's threat "If you're not a cop, you're little people" might be an allusion to Deckard being created solely for police work.
• Deckard's eyes glow (yellow-orange) when he tells Rachel that he wouldn't go after her, "but someone would". Deckard is standing behind Rachael, and he's out of focus.
• Roy knew Deckard's name, yet he was never told it. Some speculate that Deckard might have been part of Roy's off-world rebellion, but was captured by the police and used to hunt down the others. In that case, Bryant is including Deckard among the five escaped replicants.
• The police would not risk a human to hunt four powerful replicants, particularly since replicants were designed for such dangerous work. Of course Deckard would have to think he was human or he might not be willing to hunt down other replicants.
• Gaff seems to follow Deckard everywhere - he is at the scene of all the Replicant retirings almost immediately. Gaff is always with Deckard when the chief is around. This suggests that Gaff is the real Bladerunner, and that Deckard is only a tool Gaff uses for the dirty work.

The "AGAINST" case

• A major point of the film was to show Deckard (The Common Man) the value of life. "What's it like to live in fear?" If all the main characters are replicants, the contrast between humans and replicants is lost.
• Rachel had an implanted unicorn dream and Deckard's reverie in BRDC was a result of having seen her implants. Gaff may have seen Rachael's implants at the same time Deckard did, perhaps while they were at Tyrell's.
• Could you trust a replicant to kill other replicants? Why did the police trust Deckard?
• Having Deckard as a replicant implies a conspiracy between the police and Tyrell.
• Replicants were outlawed on Earth and it seems unlikely that a replicant would have an ex-wife.
• If Deckard was a replicant designed to be a Blade Runner, why would they give him bad memories of the police force? Wouldn't it be more effective if he were loyal and happy about his work?
• Deckard was not a replicant in DADoES, although he has another Blade Runner test him at one point just to be sure.

Mercilessly copied from the Bladerunner FAQ; http://www.faqs.org/faqs/movies/bladerunner-faq/