How do smileys fit in to the landscape of written punctuation?

Smilies, emoticons. We all use them, well, maybe you don’t, but the rest of us do. And while I love the non-parsable little guys I’ll be damned if I know how they fit into the rules of punctuation. Actually I’m pretty sure they don’t.

Ok, so they don’t have any grammatical function in a sentence, fine. But how do I punctuate them? Do I finish my sentence with the traditional punctuation and then leave a smiley or do I use a smiley in lieu of the conventional sentence-ender? Because it looks weird with both. Is the smiley part of the sentence? Is it it’s own weird little subjectless, predicateless sentence? Should punctuation follow it? Does the way I punctuate it change if it is in the middle of a paragraph verus the end of one? What if the smiley is represented graphically rather than with punctuation?

Please. Write me some rules of punctuation so every impish underscore isn’t accompanied by a minor attack of academic ambivalence. :(.