Rowling "is determined the latest instalment will not be developed further. She says at the end of the story, written on both sides of an A5 storycard: 'From the prequel I am not working on - but that was fun!'"
Other writers, including Sebastian Faulks, Nick Hornby and Tom Stoppard have also donated items for the auction.
I guess I'm glad that she's doing these things for charity, but I do think it's pretty rotten that she won't allow them to be shared.
[Update: The Potter prequel is on one card, not thirteen (thirteen authors total have donated the cards.]
[Further update: More here].