The concept of webcasting has been around for a long time, but it's never achieved any kind of critical mass. I think that there are two reasons for this.
Webcasts are a massively huge PITA to make. I dabbled in them once - I know.
The audience can't multi-task when they have to look at a screen.
Seriously - that's it. That's why audio podcasting will always eclipse any type of video casting. Think about the places where you see people listening to audio players. It's on the bus, walking down the street, exercising and at their desks. None of those situations are conducive to carrying a little screen around with you.
From the podcaster's perspective creating a web cast is whole different kettle of fish from creating an audio show. Not only does he or she need to become a video editor and expert, but a set has to be created somewhere in the house or office that looks halfway decent to the viewers.
Webcasts have their place for tutorials and showing live events, but as a frequently created piece of citizen media? I think not.