[kindju]

Unless Villain is completely retarded, this is never a bluff. I guess it all comes down to how we define unknown villain and his likely range. After his river raise villain is actually not completely "unkown" anymore because he just took a pretty weird line. So now is it the "heeeha I hit 2 pair on the river let's get all in!" or the "ahah I've totally fooled you with my slowplayed straight (set) now it's finally time to give me your monies!"?
In my experience type 2 is more common than type 1 and I'm not sure a lot of guys will slow play K9,Q9 until river.