Best Format for Video Podcasts - Why Not Flash Video?

Given that Flash video is probably the most ubiquitous (and a very efficient) form of video on the web, why can't iTunes and the iPod play Flash video directly? I know of people who publish popular podcasts that have switched to Flash format in order to conserve space and bandwidth. As a result, their podcasts no longer work through iTunes, and of course that means I can't take the episodes with me on my iPod either.

As I am also interested in starting a podcast, I'm wondering if there is a solution to address this problem. That is, is there a video format which iPod and iTunes can play that's as efficient (i.e. creates files a small) as Flash?

Any help or insight would be appreciated.

-Steve