Gamasutra have a mention of Blizzard's team structure, which gives away that there's an unannounced game:
"Our global headcount is 2,700," said Pearce, "And most of that is customer service for World of Warcraft! In terms of development staff it’s probably around 350. World of Warcraft is about 135 people, 40 for Starcraft II, 40 for team 3, our cinematics team is about 85 guys. Then there’s sound and Q/A and that sort of thing."
When pressed for details regarding the new project, Pearce was cagey. "Team 3 is working on something really awesome. I can’t give you any hints, but it’s totally awesome."
The conclusion to jump to is that it's Diablo 3. But couldn't (shouldn't?) Blizzard be developing something new?