There are two routes they could go for a movie, live action, or CG. If they go the former, it'll likely turn into another Hollywood-ized travesty, if the latter, Blizzard could do the majority of it in-house meaning there's much less chance some random schmo can mess things up.
A few of my friends and I have been saying it for years, but Blizzards cinematics have been near-movie quality for a long while now (just look at WoW's trailers, or Diablo 3's cutscenes), and it wouldn't be a stretch to see them make a full length feature film. Even better, they wouldn't be the first game developer to venture into movie making, so hopefully they can learn from other developers' mistakes(I'm looking at you, Final Fantasy: The Spirits Within.........)...