I don't doubt the current version has been around for a while since I seem to recall it from sometime before, but it has been appearing only recently as much as it has, and has appeared in posts which it didn't previously, so something was clearly changed such that it has replaced the orginal emoticon, which was from the same set as these two: :badsmile: :laugh:. In my experience, it also appears in the first preview of a given post even if one has smilies disabled.