You see, back then we didn´t have a little thing called the "internet" to spoil things like this. As I remember this was more or less kept a secret and neither me nor any of my friends knew about this before seeing the movie. Also I knew nothing at all about Vader´s revelation to Luke and that goes to show you that things were kept under wraps pretty tightly as long as they could by Lucasfilm. But then again I saw ESB on the very first day so I don´t know how things became when the movie had been shown for a few weeks. Then I guess those secrets had leaked out (Homer Simpson style!) pretty much.
