We're still without a date for Diablo III as of yet, but we do know about a way you'll be able to get it for free upon release.
Blizzard co-founder and president Mike Morhaime announced during the BlizzCon opening ceremony just now plans to launch the World of Warcraft Annual Pass. Available as of now on the WoW website, an Annual Pass is essentially a commitment to a one-year subscription to WoW that can be billed monthly.
There are several benefits to this, the big one being a free digital copy of Diablo III upon release. This isn't a trial or anything like that; it's the full digital game.
Even if you're not interested in Diablo, there's a reason for WoW fans to consider an Annual Pass. Not only will they get a Tyrael's Charger mount, Annual Pass members also get guaranteed access to the beta of the next WoW expansion, Mists of Pandaria.