I've seen X3 twice, and The DaVinci Code twice, and at all the showings, they showed the Superman trailer, and there really wasn't any reaction. I was paying specific attention to see if there would be, and there wasn't. The biggest reaction I saw was for Click, and POTC2. There was some positive reaction from the crowd for Ghost Rider and Nacho Libre.
X3 illustrated that people want to see major action in a film with superheroes fighting supervillains to complement a good story. Superman Returns, while it may have a solid story, is going to lack that superhero/supervillain battle, and that will possibly hurt it with the mainstream audience and at the box office. I'm confident that the film will probably pull a profit, but will it be the biggest film of the year? That I doubt.
I could be wrong, but I think a lot of the mainstream will walk out of the theatre thinking that this is a rehash of the original film, and this view could spread through word of mouth. The mainstream audience who is pumped up from seeing BB, the Spiderman films, and X3, thinking they will get action along those lines will be disappointed when seeing SR, and seeing that Superman never throws down with anyone. IMO, this is a ball that was dropped with today's technology that could have really been impressive on-screen, and may lose a lot of the mainstream audience.