All,
Have been thinking about his one myself following a recent trip on a BMI A321. I initially though that AB might have hit the nail on the head. However, I don't think that the nacelle reaches a great enough incidence for the airflow to separate significantly. However, at incidence, cylinders shed vortices from their upper surfaces, and a guess of mine was that the fins may be present to modify the form of these vortices.
Otherwise, they may be present to control some flow characteristic due to the interaction of the fusalage/nacelle/wing in that area. Either way, i'm going to quiz one on my departments academics to see if they can enlighten us!
Regards,
Cuban_8