Hey Tom,
When I've seen this come up, it's usually because of a mis-match between what Rails is setting the x-sendfile as, and the file path that apache server is serving it up as. You would see errors in /var/log/apache2/errors.log though.
From memory, apache uses the full path, and resolves symlinks, so would be like: /srv/app/releases/yyyymmddhhmmss/public/path/to/something. That's why there's the entry like:
configure :xsendfile => {:x_send_file_path => rails_root}
That might depend on the path you are using for send_file though.
Let me know if that helps!
- Josh