Legendary Pictures (yep, the same folks we know are working on the World of Warcraft film -- though, no, we don't have any recent updates on that project). How did this rumor get started, you ask? Well, if you check Legendary's home page, you'll see a project called "Diablo" listed under their in development section (in the "Films" section on the lower left, you have to scroll down a bit to see it). And how do we know this is Blizzard's Diablo and not something else? Blizzard holds the trademark to the name "Diablo" for films and games, and they've taken steps to protect the trademark in the past -- the Vin Diesel movie A Man Apart was originally to be called El Diablo. However, Blizzard sued, claiming they trademarked the title and intended to make a movie based on their game, and the title was changed. And since Blizzard has already partnered with Legendary to bring World of Warcraft to the big screen, this seems plausible, though I imagine Blizzard is going to be saving any big announcements like this for BlizzCon.
Update: Legendary has removed "Diablo" from their in development section. A mistake, perhaps?