Interesting theory. The only hole in that is I would think it would be difficult to implant memories into someone who looked different than their memories - it would be like waking up one morning, looking in the mirror, and you look completely different.
Personally I would keep it simple: Deckard is a replicant of Deckard - either killed in the line of duty or actually retired. Gaff used to be Deckard's partner (hense his knowledge of Deckard).
I guess the other possibility is the real Deckard was killed and they transplanted his memories into a replicant (maybe it's something they do for certain people - I bet Tyrell gets a replicant).
See? Isn't this speculation MUCH better than getting a prequel and having it all explained?
