Blizzard is offering an Annual Pass for WoW. It's essentially a contract that if you subscribe for the next twelve months, you get Diablo III free, a mount, and beta access for Mists of Pandaria. You still pay monthly (or whatever your current payment plan is), but you can't cancel for a year. Or possibly you lose D3 access if you cancel.
I picked up this option as soon as I saw it. I'm going to play WoW for at least the next four months until I beat patch 4.3, if not longer. I'm also going to pick up Diablo III when it comes out. There's pretty much no downside for me, as far as I can see.
This promotion is, of course, a preemptive strike at Star Wars: The Old Republic. To staunch the bleeding from the number of subs who cancel WoW in order to subscribe to The Old Republic.
But perhaps more importantly, it is a preemptive strike at Diablo III itself!
The thing is that, with Diablo III, Blizzard is making a competitor to its own game. A lot of people will move from World of Warcraft to Diablo III. By allowing people to avoid having to choose between D3 and WoW, Blizzard may get the best of both worlds. People stay subscribed to WoW, maybe logging in once or twice a week, while spending the majority of their gaming time in D3 for a few months.
The web of social ties that make up an MMO stays stronger, doesn't have as many holes from people who have stopped subscribing because of D3. Maybe they don't log in as much, but they're still there.
I think that this is a fairly strong indication that when Titan comes out, a Titan subscription will include a subscription to WoW.
It may even be the first step to one day having a "Blizzard subscription", where you get access to all their MMOs, and free copies of all their single-player games when they come out. Other MMO companies have done single subscriptions to multiple MMOs, but I don't think anyone has yet added single-player games to that. I think Bioware is probably the only other company that could do so.