Last week was Banned Books Week, and I celebrated by reading one of the books in the list of the top ten books challenged in 2011: Brave New World by Aldous Huxley. Fair warning: This review does contain some spoilers!
Some friends and I had a discussion the other day about why there is almost always at least one classic on the list every year. I'm pretty sure it's because most instances of books being challenged occur in the schools -- patrons of public libraries don't typically challenge books being on the shelves, but parents and other adults are constantly trying to dictate what kids ought to be able to read.
They usually challenge books that are being taught in the classroom, or contest whether certain books ought to be accessible in the school library. I'm sure that most, if not all, of the classics that make it onto the list were part of the curriculum.