I actually wrote my dissertation on the intersection between the public and private sphere (based on Trollope's Palliser Novels in case anyone is interested). It strikes me however that there is something much more interesting going on now with Web 2.0. It started with mobile phones and has just spread everywhere. I haven't quite worked out the theory yet. But at the moment my thinking is something like this.
There is nothing like spheres anymore because that relates to a spatial thinking which is no longer relevant.
http://www.gravity7.com/blog/media/2005/10/blogging-as-public-and-private.html
Instead of which the relevant metaphor is more about volition and consciousness. It's about when you want to be seen and noticed and when not. So the woman I saw brushing her hair whilst waiting at the Bus stop would chosen that that was a time when she did not want to be noticed. It didn't matter that there were people around her, she had decided that she didn't want to be part of any interaction and so therefore her actions should not be considered as in any way part of anything. The people who shout intimate things down their cell phones on the train have decided that they are not part of the social environment around them. As the post above says in a phrase I like, it's about "presence negotiations" but also about presence volition. 