default role for new users

 function _user_authenticated_id() {
  return db_result(db_query("SELECT rid FROM {role} WHERE name = 'authenticated user'"));
}

Maybe you can change this line in user.module to add another role. Sorry I didn't take time to reply exactly to your issue, but it shouldn't be so difficult to modify this in user_configure_settings().

I don't see why we'd need that in core. You can easily implement this in a contrib module by implementing hook_user('insert').

There is a module that implements this now: http://drupal.org/project/rolesignup

It's not exactly as the original requestor asked, but it works well for some situations.