Pottermore, a mysterious website bearing J.K. Rowling's name. The Atlantic reports that a leaked memo making the rounds spells out the truth: Pottermore will be an online game of sorts.
The memo "suggests that [Pottermore.com] is a sophisticated online game that contains clues to prizes that are hidden in the real world. These are an unstated number of magic wands secreted in Britain and America, and possibly other countries."
This sounds more like an ARG -- alternate reality game -- than an MMO, although we're not prepared to rule out the possibility until all is said and revealed. The article mentions that this could be a marketing tool tied into Pottermore instead of Pottermore proper.
Fans who'd love to see a Harry Potter MMO still have hope that this could be the real deal, but we won't know for sure until tomorrow's announcement.