Constantly! I try to do it only when it's just me around, but I know that roommates and boyfriends have caught me here and there. Especially in the car too, because no one can hear me. If I'm really upset or extremely focused, my mental mono/dialogue will start escaping through my mouth in public, until I catch myself...sometimes I think I should just start wearing a non-functional Bluetooth, so people will assume my comments are directed at that, rather than myself.
Also, when I need to be organized (I was a waitress in the past, and I'm a nurse now) and do some extreme multi-tasking, I chatter lists to myself, but that's pretty well-accepted in both fields.