I believe the D3 date is, unfortunately, unrealistic.
I got the sense from Blizzcon that they are no where near finished with the content or the games internal systems. The fact that they were unable to answer many questions at the Q&A except by saying the "we're still thinking about it" standard response was discouraging. Add to that at least 6+ months of alpha/beta'ing before release. Q4 2011 is too soon; a release one year or so from now? Not going to happen.