We've discovered that Blizzard and/or the Orange County Visitor's Center has reserved many rooms in the hotels surrounding the convention center specifically for BlizzCon guests. In order to claim them, though, you have to call the Visitor's Center directly. If you have a specific hotel in mind, you can ask for it, but they can also hook you up with other hotels that fit your price range. There's no cost to reserve a room, and you have until Sept. 1 to cancel the reservation with no charge. You just need a credit or debit card to have on file.
The number to call for reservations is (714) 765-8868. They're open normal business hours, 8:30 a.m. to 5 p.m. Pacific time.