It depends.
In the original story, no he isn't. I don't think Mr Dick even hints towards it. However, my copy of "Do androids dream of electronic sheep" has been missing for longer than you've been alive Lain - so I may be mistaken
In the original cinematic cut of the film? No. It's simple action/adventure/love story and although the hints are there, they're hidden away and you're not meant to think that he is at all.
In the Directors Cut, yes! The scenes added in point to it almost definitely. While in the original cut you could argue a case for him being a replicant - you'd be really clutching. The Directors Cut shows no background to Deckard, he simply "appears". The unicorn motif throughout also shows that there is a link between him and people who are aware he's a replicant. There is also the tune he remembers, but it seems an enigma to him how he knows it; the obsession with photographs that all replicants have and his ability to defeat the other replicants. There are more hints too, but altogether, they make it very VERY clear he is.
I've not seen the ultimate cut or whatever it's called. I've seen the film far too many times and it's hard now to get the time and patience to sit through it yet again...