I have to say, that I was always in favor of fanfic; after all I got my start writing it. Darkover, Star Wars, Cthulhu Mythos, a little Indiana Jones... Like MZB, I encouraged it. Until Le Debacle. Short form was that Marion read a piece by someone in the Darkover fan circles, liked how the author treated Lew Alton, and offered what was the usual arrangement, an acknowledgement of where the idea came from in the book that Marion was going to write. Because, face it, it isn't the idea that is the work, it is the execution of the idea. There was no offer of collaboration. The author in question demanded full collaboration and payment, and threatened legal action. As a result, Marion abandoned the half-finished book and destroyed the manuscript, which now will never be seen. A doubly damn shame because this was one of the last things she was writing before her series of strokes started. And at that point our mutual agent decided that Fanfic was Dangerous, and should be discouraged. This made me sad, but the mess with Marion made us all super cautious.
Fast forward to recently when Russ (my agent) got Cory Doctorow as a client. Now, as we all know, Cory is Awesome personified, and is very, very much in the corner of the fanfic writers among us. Cory persuaded Russ that it was a very, very good idea to allow Creative Commons licensing of fanfic, that fanfic is good for the author, good for the fanbase, and takes not one thin dime out of anyone's pocket. As a result the only time Russ will turn into the Great White Shark is when someone is trying to make a profit out of fanfic.
I still can't read fanfic of my own oeuvre, which makes me sad. But as the lawsuits against Stephen King and Jo Rowling have proved, you have to be able to rigorously prove you never saw someone else's unpublished stuff when they sue you for "stealing their idea." But I am very, very glad that people can write their fanfic and share it now.
And hey, Atlantea, tell Bob Shroeck he can go ahead and write the first segment now!
Now there are other writers who think that fanfic is taking some of their income. Uh, no. No one is ever going to write as fast as readers can read. All fanfic does is keep the flame stoked during the wait for the next book. But you can't convince these people, I know, I've tried.
There are other writers who don't want anyone messing with their world and characters (some have even described a feeling like "being raped" when they discovered there was fanfic about their stuff. On the one hand, I kind of sympathize...but there is no way short of inventing mind control that any writer is ever going to be able to control what happens to his book, world, or characters once the manuscript leaves his hands. So IMHO, I think you ought to just pull on your adult pants, face up to that, and wave goodbye to your book as it heads out into the world. You did your best by it, you tried to make things clear, you gave it the best start in life that you could, but it's on its own now, and you can either make yourself crazy by trying and failing to keep control, or you can do the smart thing and go on to the next book.
Whew. Sorry that turned out long and OT.