Anyone used acts_as_flying_saucer? problem with pdf_file

Tracked this down to a problem with x_sendfile - i needed to set it to
false in order to get it working. Here's my current call:

  render_pdf :template => 'school_subscribers/show_pdf',
             :send_file => { :filename => filename,
                             :x_sendfile => false,
                             :disposition => "inline",
                             :type => "application/pdf"}

I don't know why x_sendfile is going wrong - it misbehaves the same way
in my local single mongrel setup, my local apache&mongrel setup and our
staging server which is apache with multiple mongrels. I don't know
much about x_sendfile but even my much more knowledgeable colleague is
stumped.