Blizzard CEO Mike Morhaime has confirmed that there will be an expansion for Diablo III coming. He announced this during the Activision-Blizzard investor call covering the fiscal quarter ending on the 30th September.
According to Morhaime, the expansion is currently being planned. There is no release date set, but the CEO did state that the top priorities for the expansion are the gameplay and its overall quality.
Knowing Blizzard, that means the developer will be adopting the "it'll be done when it's done" attitude towards the expansion.