Having worked at two shooter bars which both include the slippery nipple as a part of their menu, I have this to add.

Although a part of the same "up-and-coming" chain of bars, the first place just used sambuca with Irish cream floated on top, while at the second place we used to put a cherry in the bottom of the shotglass. This was explained to me when I started off as "If it hasn't got the cherry, it's not a slippery nipple, it's just slippery."