I've often wondered that. The wife seems to think that boys look up to their Dads and will share interests no matter what they may be. I think there are several factors that have a pretty strong influence on kids including, but not limited to parents, television, marketing tie-ins (happy meals, cereal, etc.), and school.
School seems to produce the most "misses." By "misses" I mean things kids will feign interest in to fit in to a particular group. I see it all the time with him; he'd tell me how he played "Dragonball Z" during recess, but shows no interest in it at home.
This is where I expect the "How can you tell if he's feigning interest around you?" question. To which I can only say, when you know somebody as well as a parent knows (or should know) a child, you can tell. For instance, I like baseball. My son will play along (not that I push him to) and go to the occasional game or watch half an inning on television with me, but I can tell he's just humoring me.
You mention Pirates, Pixar, and Iron Man. In our house, those seem to cycle with the advertising. The mainstays have been Star Wars, Indiana Jones, and comic book heros in general. I think there are two reasons for this, at least in my son's case, that cannot be tied to outside influence.
1) If you're going to run around the house beating up bad guys, you're going to do with while humming a John Williams score. The music really seems to stick more than the actual movie when it comes to re-enactment and action figure play time.
2) Curiosity. With Star Wars there are six movies, a cast of thousands, and an infinite expanded universe. That's enough to get any kid's imagination going for a very long time. With Indiana Jones, the movies are based in reality. I love being asked history questions, and making trips to the museum or library to expand the little person's understanding about a subject they are now passionate about because a movie sparked his imagination. This was true with Pirates as well (not Jack Sparrow, etc. but he became interested in learning about "real" pirates). Comic book heroes provide kind of the same amount of unlimited storytelling potential I described about Star Wars. (Though, you might count that as parental influence because I don't think many parents have such an extensive comic collection or enclyclopedic knowledge of all things Marvel and DC such as myself.
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Holy crap that was long. And upon reading, not totally coherent. Oh well, I'm not re-writing that. I hope some of it makes sense.
edit: Man, BM, that's some story. I had a falling out with my parents and sisters and really to talk to them much. You're making me want to pick up the phone, man...
Short Round wasn't even in KOCS! 
