My wife has a ~5 year old cousin that's eyeballs deep into that stuff. My wife and I both find it nauseating. I'm pretty sure we'll figure out a way to train any girls we have out of it. My wife's boss had a good suggestion: you can't be actively negative about it, but you can hint that girly princess fairytale stuff is for babies, and they'll naturally push themselves out of it so they can feel older.
I'm guessing a lot of it is social circles and TV. The cousin even has Barbie Princess video games :barf: