I think Aaron Goldman has it right in this MediaPost article. Pre/post-roll ads probably aren't the right vehicle to monetize online video. It's like applying the previous generation's solution to today's technology.
The scrolling ticker approach he mentions is a far better solution. It still seems less than perfect, but it's definitely more effective in the "lean forward" online environment, as he characterizes it. In fact, I like the ticker approach so much that I suggested it as a possible solution for DVR ad-skippers here. And again, who says TV shows have to remain 30 or 60 minutes long? Why not have 3 in an hour, especially if 22 minutes of real content is all you normally get in a half-hour slot?