J.K. Rowling has handwritten an 800-word story about James Potter and Sirius Black that takes place three years before Harry Potter was born. The story card was recently auctioned off for charity. Unfortunately, the story is not from a prequel that Rowling is working on. :o(
You can click here to read Rowling's account of writing the story and (hurray!) here to read the story. I've read the story. I don't think it's anything spectacular. But it was great reading something new from the Harry Potter stories. It was wonderful returning to the world of Harry Potter. I want more!
Thanks to Wizards Wireless and Publishers Weekly Children's Bookshelf for the links!