it was initially expected to show up by the end of the year, Blizzard has now officially announced that Diablo 3 will be pushed back to early 2012. As you already know, beta testing has been underway since September 20.
The delay doesn't come as a surprise if you consider that the first impressions of beta testers weren't all that good. The game looks good but according to those that had a chance to try the beta out, it still needs a lot of tweaking. Blizzard will also extend the beta testing and add more testers.
Blizzard didn't give any specific date and current official schedule is early 2012. Mike Morhaime, CEO and Cofounder of of Blizzard Entertainment add that the beta testing is going very well and despite the temptation to launch as soon as possible, they don't want to waste those many years developing Diablo III and release a game that is "almost ready".
You can check out the offical press release here