Mercedes Lackey has announced the following on her blog:
News: Concerning Fanfiction:
As you folks already know, my agent, Russel Galen, has in the past been opposed to fanfiction. However, he is also Cory Doctorow's agent now, and Cory is a persuasive little gnome.
As a result of this, I am happy to announce that we are officially permitting fanfiction to be licensed as derivative fiction under the Creative Commons umbrella...
Thanks, Cory, for helping Mercedes and her agent recognize that fans' reaction to works can (and already do!) legitimately include creative responses. We don't think we need her permission, but we're always happy to have her blessing--and as fans of hers, we're happy that she's happy! :)