With Let Me In holding steady at 100% on Rotten Tomatoes and some awards consideration could be merited for the Swedish vampire film remake. This would be the first time since Silence of the Lambs Won Best Picture in 1991 that a horror film could be up for the top prize. Of course it bears mentioning whether or not Silence of the Lambs is indeed a horror film. Portions of Lambs suggests that Lector is almost a demon of sorts, enough to qualify as horror?
If not, then you have to go all the way back until 1973 when The Exorcist was nominated for Best Picture. Rebecca won Best Picture, but I would classify that particular title as suspense/thriller. Rebecca was taut, but never really crossed the threshold into frightening. Psycho was nominated for four Academy Awards (2 for technical, 1 for acting and 1 for direction) and, again, that is up for your interpretation of that film is in fact horror.
With the genre being so hotly-contested as to what is thriller, suspense, and actually horror, could we see a horror film win Best Picture?