From Diablo Wiki
Blizzard has frequently expressed interest in adapting their game properties into feature films, but nothing has ever gotten further than wishful thinking.
Jay Wilson, the Lead Designer of Diablo III has talked a lot about "finishing" the story line of the game, but also about how much can be done afterwards. Hints about a movie has reappeared, and seems even more likely now with the WoW movie getting closer to production.
November 15, 2005
A short news item on CVG excerpts a few quotes from Blizzard President Mike Morhaime in which he talks about hopes for future films in the Blizzard worlds. The interview originally appeared on the Unofficial WoW site WoW:IncGamers released at BlizzCon 2005.
Blizzard Entertainment President Mike Morhaime has spoken of how he "would love for us to release feature-length films in each of the worlds", letting slip a desire to see Warcraft, StarCraft and Diablo make the move to a new medium in the silver screen... Explaining the grand reasoning behind this 'love' in a recent interview, Morhaime has said he thinks that "the universes are rich and strong enough to support that kind of thing" and adds that in his mind it would "enhance the connection that players have with the game". Morhaime also spoke of a desire to see the mainstream appeal of videogames expand and about how he'd like "more people outside of the core audience" to be "familiar with our gaming world".
This is obviously just a "we hope to do a movie someday" item, rather than anything more concrete, so don't hold your breath. After all, all the way back in 2001 Blizzard won an injunction preventing New Line Cinema from calling an upcoming film "El Diablo," a ruling based on Blizzard's previous copyright of that title for an upcoming film. The movie was renamed and released in 2003 as Man Apart. Wanting to make a movie, or copyrighting a title for one, is obviously not the same thing as actually producing one, sadly enough. (Matt Samia of Bliz cinematics was interested in doing that all the way back in 1999.