Joining a list of string subtype instances usually
results in a single string instance:

  >>> class mystr(str): pass
  >>> type("".join([mystr("a"), mystr("b")]))
  <type 'str'>

But if the list only contains one object that is a
string subtype instance, that instance is returned
unchanged:

  >>> type("".join([mystr("a")]))
  <class '__main__.mystr'>

This can have odd effects, for instance when the result
of "".join(lst) is used as the returnvalue of a __str__
hook. "".join should perhaps return the type of the
joining string, but definately vary its type based on
the *number* of items its joining.