"deja vu" is supposedly caused when the brain (try to follow me here, i know its tough, but i relly did read this) "short circuits" and loses its place, and then picks up whre it left off.
ex.
youre staring at a can of pop, and then your brain pauses, fixes its error, and goes back to recognizing the pop can. you only looked once, but your brain saw it twice..
I think it is superstitin and lack of explanation that lead people to beleive that they have dreamed the situation before since it is impossible at times to have actually expirienced it.. which is why you often here them say, "Hey wait! i dreamed this!". And they do not recall the dream beforehand.
i mentioned this a loooooong time ago on another thread and someone (sorry, i can't remember who) found more information on it. perhaps they can give it here agian.