Good article from The Boston Globe Magazine by Tracy Mayor on fan fiction and the difficult copyright, trademark and business issues raised when fans like a work so much, they create unauthorized derivative works.  Quotes from J.K. Rowling’s publishers:

“From Scholastic, her US publisher: “We are very appreciative of Ms. Rowling’s fans. We are only concerned with anything that denigrates the property or is disparaging of it.” From Warner Bros.: “We object only to fan fiction that is offensive to children, meaning anything sexually explicit, violent, or profane.”

From Rowling’s London-based agent, The Christopher Little Literary Agency, this comment: The author “welcomes the huge interest that her fans have in the series and the fact that it has led them to try their hand at writing.” Therefore, the agency says, Rowling and her agents act to enforce her copyrights only when there is a commercial angle, when work purports to be written by Rowling herself, or “where the fan fiction is pornographic or inappropriate for kids.” “

The illustration is by Glynis Sweeny, from the Globe article.