Waterstones (the UK bookstore) recently ran a promotion called "What's your story" where they got people to submit a postcard with a short story on it, and they will be turning the best into a limited edition book.
In total they had 4522 entries, and some of those were from celebs - including JK Rowling.
The postcard book is going to be made in a limited run but they have posted the celebrity cards on the What's your story website so you can have a read now.
JK's story claims to be - "From the prequel I [JK Rowling] am NOT working on", and is a snippet of action from a young James and Sirius.
To read JK's "prequel" you should go to the What's your story website, then press authors -> JK Rowling (unfortunately it's made in flash so I can't send you directly there).