Robyn Tippins wrote a short article on RSS Applied that states Vidcasters might be able to command 70% of the available podcasting advertising dollars.
I think that's likely true on the surface, but I agree with Robyn's warning that Vidcasting isn't a prolific as podcasting and has the associated smaller market. While podcasting is more definitely "here", Vidcasting isn't quite here yet.
We tried Vidcasting for a while (admitteldly, half-heartedly, but we did it) and found out that it was about 2,000% more work than producing our audio podcast, so we dropped it. Putting a Vidcast together not only requires all the associated research and setup as an audio podcast, but even more work is required to set up the set, lighting, clothes, different equipment...yuk - totally took away from the fun of the show for us. Viewers are also another matter. As Robyn points out, many people don't have video-capable portable players so even if they wanted to partake in a Vidcast, they couldn't.
Vidcasting becomes more mainstream every day and I think it will be viable within 2 years at the outside and probably closer to 1 year.