If you're a registered World of Warcraft player, and are willing to commit to a 12 month contract to the game, you can get Diablo III for free. In one of the more confusing offers I've seen, Blizzard seem to be rewarding ongoing loyalty to WoW players, rather than actually trying to get new customers, as this is an offer open only to those who have a registered WoW account on or before the 18th October 2011. If that's you, and you're willing to add twelve months to your sub, then not only do you get Diablo III whenever it happens to release, but access to the beta for Mists Of Pandaria (still can't believe that's really a thing) and a new walkie/flying mount, Tyrael's Charger.
The caveats, they are many. For instance, if you cancel your 12 month sub for any reason, whoosh, your copy of Diablo III will be gone. After the year you can cancel and keep. However, you don't have to pay for the full year up-front - it's done in monthly installments. And once that Panda-infested beta is open, you have guaranteed access.
If you want the Collector's Edition of Diablo III, they've worked out a way you can add your code to your free version and get all the benefits added. But, um, why would you do that? And that mount, who can get up to 310% speed while flying, will be added to your account once patch 4.3 is released.
And yes, reactivated WoW accounts are eligible, but multiple accounts won't gain you any more. To find out the rest of the details, head here.
Of course, what you're actually getting here is Diablo III free if you spend $180/€156/£108 on WoW. So this is pretty much just an offer for those who were going to be playing WoW anyway, in a year when it's facing some extreme competition from newer games. Clearly they want that up-front commitment to stop your wandering off and offering your monthly MMO tithe to another. If you were going to stick around anyway, awesome, free game, beta access, and a stupid pretend horsey.