You know, I've had a different experience with this than some posters.
At the first preschool I worked at, several of the boys were very familiar with Power Rangers and the Ninja Turtles (early 90s). They did a lot of pretend play around these shows, and sometimes didn't pull their punches or kicks. In the mixed group, it created a very uncomfortable environment for some of the other kids because of how these children were using their bodies. It was also hard for other children because of some of the violent language that was coming out during those times, so even if no hitting was going on, it did affect how 'safe' some of the children felt.
My main problem with these shows is that they are 'might makes right' types of stories. The problems are never solved with discussion or words, only with fighting. I don't like the lesson that teaches. Sure, the 'good guys' win, but they do it by defeating others physically.
My son is five and we don't watch it at our house. He's not feeling particularly deprived... and I know as a boy, he's going to play guns and fighting... but I like that he isn't locked into a character that can only solve their problems by hurting others. (It's also just not part of his temperament-- he doesn't even like to watch the old Three Little Pigs cartoon-- the Big Bad Wolf chasing them feels 'scary' for him. I try to respect this. He's very brave in the ways that matter.) There are some very good articles on children playing at violence; I know that for many children this is a very real part of their development and there are ways in which teachers can facilitate making this play 'safe' for the group. I just don't think we need to offer models for violent play, either. They do pretty well with just their imaginations.