Some quite intersting and thoughtful comments here.

I recall an anecdote which I think reinforces both Pip's definition and Glueman's query about 'Streets of London'. A few years ago, a singing session I was in was visited, and particpated in, by a perplexed singer-songwriter. We would say 'this is a Robert Burns song' (or whatever), and eventually he burst out 'don't you do anything except cover versions?'

Actually that was quite a good question. when does a cover version (of 'Streets' etc) become 'actually, no we are absorbing songs into an oral tradition and thereby passing them on as folk songs'? (Which is what we like to think we were doing).

I think it comes to what Pip said about there only being one way to play/hear Streets of London and what Jack C said earlier about guitarists learning verbatim tab. If we change the tempo, key, accompaniment sufficiently, we are starting a song off into the oral process (even if its something contemporary - I've heard a folk club version of David Gray's 'Babylon'). But, if I learn the tuning, fingering and Tab for say, Martin Simpson's arrangement of Polly on the Shore (which is a traditional folk song), I'm not really passing on an oral tradition if I play that arrangement exactly as written (if only...).

How does that sound?