An intriguing idiom. It is commonly accepted that no one is perfect, yet somehow unknown people are awarded perfection! Or perhaps it means that they have not yet been contaminated with the sin of knowledge - their knowledge of you or your knowledge of them. Now that several hundred million people have had their nervous systems extended via the internet, the term may take on a different dimension. Of course, the big problem with any e-communication is the lack of context. Someone in your community provides lots of context while someone chatting or blogging offers little or no context. After a few months of interaction a sense of the other person develops, certainly colored by what is hidden from view (or openly displayed), but really not much different than getting to know someone in physical reality. There have been instances of fictional people being portrayed as real, with cruel results following the breaking of the illusion. (just a note to anyone who my be confused as to who I am: read my bio in the sidebar)
Overall, most of the people I have run across in my internet wanderings have been PERFECT strangers. Intelligent, interested in an exchange of ideas, open and outgoing, yet not intrusive or annoying. If that isn't perfect behavior, it is as close as you can get.
Of course there are plenty of flamers out there - especially in politics and religion - and the accepted meaning of "perfect stranger" is a completely unknown person; yet sometimes it seems as if the world isn't quite so bad, after all. Maybe I've been lucky, maybe I've just been too timid, maybe the imperfect strangers don't read blogs like this...