Apparently, that particular emoticon indicates "tongue sticking out, cheeky/playful." It is also used in reference to blowing a raspberry, which is basically making a noise signifying derision, real or feigned, made by placing your tongue between the lips.
It's a "sticking your tongue out" face. :-p I use it when I'm bein' a "wisegal," or when something makes me wanna puke. It goes with sarcasm/joking around, or feeling nauseated/sickened -- those get a :-p from me... :)