I have been thinking about this some more and I agree that the promotional material showing Yoda shouldn't be considered a good reason for the surprise not being for the audience. Look at the posters for The Two Towers. Gandalf was plastered across them, and it did spoil a surprise for people who didn't know the story. I would say that is much less the case with the Yoda example from ESB. Furthermore, I think the surprise with Yoda is twofold. Firstly that the little green guy is Yoda, and secondly when he lifts the X-Wing from the swamp. Up to that point neither the audience or Luke has seen the force used to lift something so big. Which is why personally, I think not only should Yoda's appearance be kept secret until ESB, but also before that we should only see large creatures use the force, and not to do anything on par with lifting the X-Wing. It spoils the scene in ESB the same way that Dooku using sith lightning spoils the scene in RotJ.