I search and research and it's true: no science fiction film has won at Best Picture (at least there are fantasy adventure films which have done it).
However, I cannot think in Avatar as an original idea. Have you seen Dance with Wolves or the Last Samurai?, or do you know about Pocahontas?
I think that the reason that the film was there at the nominations was because of two things:
1. The film broke box office records, one of the highest grossing films, in other words, it was a commercial success (due to too much advertisement and propaganda).
2. The special effects (which of course, utterly won at the Academy Awards).
However, the two points mentioned before don't mean that the film has an original or great plot.
In my point of view, I would have prefered that Up would have won as the Best Picture. Of course, I haven't seen The Hurt Locker, but it was a great movie and because it is only an animated film doesn't mean that it doesn't deserved the award (What made me have tears of joy was that not only won for Best Animated Picture, it also won as the Best Original Music Score!!!!).