BlizzCast has not been forgotten, it has just ran into unforeseen delays, but it is still in development. Here is what Nethaera commented at the official forums:
Nethaera: “A third one is in the works. We just had some unforeseen delays we had to work out. We definitely want to make these more regular releases, however, they depend heavily on getting many people and resources together.
I’m a part of the BlizzCast process (you can hear me in the second one). We have never stated this would be weekly, though we would like to be consistent on monthly releases. We’ve only released two BlizzCasts so far and we are still nailing down a process that works best for us. I personally listen to many podcasts and think they are great, but they definitely work within different constraints than we do.
We’re doing what we can to get these out to you and hoping we can just keep improving with every release of it.