LOL, you’re awesome. 
It means exactly nothing to me.
I always assumed it meant somebody typo-ed when trying for the code for some smiley.
It sounds like I need to be more careful when using smileys because not everyone agrees with what they mean!
I always thought : ) meant “just kidding” and that : p meant “I’m being cheeky / just teasing”, with the distinction being necessary sometimes.
e.g. If you’re teasing a friend about how drunk they got on a recent night out, a : ) indicates you’re joking, but it still may come across as passive-aggressive; like you’re trying to indicate your disapproval in a light-hearted way.
But if you finish the joke with : p it’s interpreted (I thought) as “just teasing”, the way friends do; you expect the respondent to come back with a tease of her own.
Oh, and I agree that the stick out tongue emoticon on SD (:p) looks misleading. It looks like someone chuckling. It doesn’t convey cheeky to me.
I think of it as sticking out your tongue, which as in real life, can either be funny or rude depending on context.