If you watch the newest version (it's like an ultimate director's cut or something), it's Ridley Scott's original vision for it. He has said in recent interviews that his intention was that Deckard was a replicant.
It's really interesting to watch thdirector's cut version, then watch the original theatrical release with the voice over. Supposedly, Ridley Scott and Harrison Ford were both vehemently against doing the voice over, but the studio forced them to do it. Harrison Ford has said that he intentionally did the voice over very badly in the hopes that the studio would decide not to use it, but they did anyway.