During an Activision earnings call earlier today it was confirmed that an expansion pack for Diablo III is currently in development. Meanwhile, StarCraft 2's Heart of the Swarm expansion will release in the first half of 2013 if everything runs according to Blizzard's plans.
Blizzard president Mike Morhaime confirmed that an expansion is on the way during his address. "We do have an expansion planned for Diablo, [but] I don't have any timeline to talk about. I think the most important thing for us always comes down to the quality of the expansion and the gameplay, so that will be a big factor in terms of driving our schedule."
Morhaime also announced that Blizzard is aiming to release StarCraft 2: Heart of the Swarm in the first half of 2013.
It was an optimistic earnings call for Acitivision, with Bobby Kotick boasting over 10 million copies shipped for Diablo III, and $841 in net revenue during Q3 2012 - up from $754 from last year. All the fine details are in this PDF .