Well, let's see... technically, Meet Dave and the various superheroes movies (Hancock, Dark Knight, Iron Man, Hulk) could be considered sci-fi, but... that doesn't seem legitimate. I suppose that Wall-E counts, but...
I'd have to say that the last real science-fiction movie I watched was Alien and Aliens back-to-back. One of the shames of my life was never having seen those films all the way through, so that was a treat. Oh, and I watched Empire Strikes Back because it was on TV, but c'mon -- who doesn't do that?