I don't think anyone realistically expected Diablo III to arrive before the end of the year. The studio had admitted 2011 was in the realm of possibility but always said that was never a guarantee, and the idea of Blizzard taking a few more months to iron out the kinks is right in line with Blizzard tradition of "when it's done" or "as close to perfect as humanly possible."
You're right to be skeptical about today's news, but what makes the news more reliable is because it's been written down in a press release. Prior to today, Blizzard hasn't committed a specific release date range for the game in official form. That's not to say Diablo III couldn't slip past early 2012, however.
"With every game we make, the temptation is always very strong to launch as soon as possible. However, we didn't put so many years of work into Diablo III to release a game that was almost ready," said Blizzard CEO Mike Morhaime in the press release.
On the plus side, Blizzard will be extending the beta test for an undetermined amount of time and "potentially adding more testers than initially projected."