Maybe for a few exceptional cases like Facebook or Twitter, but 99.99% of developers don't have near that much leverage.
In this case, for example, if Instacast isn't in the AppStore most people will simply choose an alternative "podcatcher" that is.
Honestly, most of the companies who could vote with their feet on this issue need the AppStore a lot more than the AppStore needs them.
Discoverability is a huge problem, and honestly, more than 80% of the apps aren't very good. And given that iOS apps are still in the gold rush stage, that percentage of crappy apps is growing by the day.
If a significant number of the developers who vote with their feet are part of the top 20% (for quality), and the boycott becomes more widespread, I think Apple would have to respond.
In the post-Jobs era, who knows if something like this could have an impact.
Especially the massive number of young kids buying apps courtesy of the iPod touch.
Keep in mind, 20% of 500k apps is 100k apps. Do you really think there are are that many 4 star apps in the app store?