"Alien" may be in the sci-fi horror part of the genre (so is "The Thing" and the original story it came from, "Who Goes There?) if it were separated in a sub-category but it's definitely not fantasy. There was a plausibility to the science in the film and its success depends on the atmosphere, suspense right down to the musical scoring. The whole of this tour-de-force of suspense and tension could not exist without the crew not immediately thinking of everyone donning space suits and then evacuating the air. The android was certainly not going to clue them in. Almost all suspense drama falls apart if you pick away at it. It's the old "don't go up that staircase, you fool, and absolutely do not open that door." "Alien" is essentially a remake of the original movie of "The Thing," sans the morphing and absorbtion element, of course, only in the future and in space. Ever notice most of the menacing aliens attack Earth aren't specifically placed in any future time but in almost an alternate universe. Both filmings of "The War of the Worlds" are framed in present day, like "it's going to happen the day after tomorrow." "Donny Darko" rather satirizes the whole formula -- I would not enjoy that film either if I took it really seriously.
In the origins of the written genre, the old handle just happened to be science-fantasy. This was the old Amazing days up through the era of the space opera. Wikipedia happens to have a pretty good article on "science-fantasy:"
http://en.wikipedia.org/wiki/Science_fantasy
Future fiction is also a bygone term except that nearly all sci-fi is placed in the future. There is socio-political future fiction like "1984" which doesn't really rely on much science, and there's satirical future fiction as in "The Space Merchants." "The Fifth Element" is a comedic future fiction if a bit heavy handed with it, also remarkable for its production design. Did not garner much critical respect but it's one of the most repeated showings on cable. I think it's because it's a hell of a lot of fun -- can't resist that Blue Diva (neither can Roger Ebert).
"Blade Runner" is most significant for its production design -- a future world still mired in the remnants of the past. It has socio-political overtones, more so in the novelette. "The Demolished Man" is also in this category. So there's all sorts of shades of gray where science isn't the totality of the basic premise.
I don't short RT for trying to close off pidgeon holes but it leaves us with a less-than-comprehensive list. Now let's see AFI to the 100 Sci-Fi and Fantasy films -- they would also be forced to combine the two.
