This is a curious thread to me because I understand the cultural implications. I've lived in Europe quite a bit (including southern Germany, which, no offense to the Swiss, is actually pretty similar, culturally) and I'd probably let dd do this if we were still living there. And I lean toward the *over-protective* side, so take that in to account.
In the US, though - no way, no how. Even at 9, socially savvy, and with great common sense... I still wouldn't do this in the US. It's a completely different animal we're talking here. In Switzerland (in which I've spent a fair amount of time), and with the circumstances, I think I would have been comfortable with MY dd doing this at 6 years of age.
The interesting thing is that in Europe (and well, around the world, actually), a lot of kids have to take public transportation just to get to school. I often saw young children (usually with a friend, not often alone, though) in their little uniforms, on their way to school on the S-Bahn in Munich. I don't think they were 6, but definitely they were 8 or 9.