This morning Rowling launched a preview page for the tantalizingly named "Pottermore" site, which would seem to imply MORE POTTER. A link (via owl) from the preview page to a YouTube video indicates the answer will come on June 24. There's a Twitter page as well.
It is possible that the Pottermore site itself IS the project, not further Potter books, nor a Potter encyclopedia project. It could be an immersive world a la Second Life, or a massively multiplayer online role playing game (MMORPG), or an inclusive fan site with every manner of feature, but Potter fan site HPANA says "it is breathtaking in scope, detail and sheer beauty."