My biggest problem with the "unicorn means Deckard is replicant" idea is that (according to one "clue") somehow Gaff knew exactly which thought to reference with his origami. Deckard talks earlier with Rachael about her memories of being young. She starts a story and he finishes it, knowing how the false memory goes. But Deckard daydreams at the piano (unless it really is a memory and unicorns exist in the BR universe) and Gaff even knows about that errant thought? Nah.
On a certain level, what's the difference? It wouldn't be a real memory, it would be equivalent to a false memory. It stands to reason that if they can implant false memories such as the one described by Rachael and Deckard, they can implant unicorns.
What I mean is, it's clearly not a memory in any way unless unicorns are real, so (if taken as proof that Deckard is a replicant) it implies that Tyrell Corp. not only implanted memories into Rachael and Deckard but potentially every little passing thought that runs through their mind on a daily basis. Which would make them (in my mind) even less human than the other Nexus 6 replicants.
I prefer to see the unicorn in a less prosaic light. That Deckard daydreams of a unicorn, and when later Gaff leaves a piece of origami to tell Deckard that he has been there, rather than it throwing Deckard's whole worldview into question, he sees it as one of life's wonderful little funny coincidences.
But as I say, the real beauty of it all is that there is no true answer. The director says one thing, the actor and writer say another, the audience get to decide for themselves. Some resolve to watch it again this weekend for the umpteenth time.
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