Spinash,
Had a chance to look at the 717 nacelle devices. Their position is, I think, a little too low to provide just for clean separation. My guess is that they generate a vortex which curls up and around the nacelle to delay separation at higher alpha. It would take a response from someone at Douglas to put this one to bed.
Then again, with tongue in cheek ...perhaps they are there just to keep the rain off the ground engineers working under the nacelles ?