If your friends and co-workers haven't figured it out yet they must have a hearing impediment bigger than your speech impediment. Notwithstanding the fact that they should already know it, let them hear it from your mouth. I am not saying that you need to stop what you are doing, gather all the people around you , and make the announcement. You can, however, do it nonchalantly. I tend to turn conversations toward talking about briefings, speaking in public, or teaching in front of a group - then I throw in the "Well, sometimes it's a bit difficult for me since I've been stuttering since I was four". Your listeners will pick up on it, but more than likely their response will be something like "Oh, but you mange very well" and the conversation will continue its course. The next time you stutter in front of them it will seem perfectly normal since they are already aware that you do.