If Apple or any other company refused to play ball on something, all of the apps under the union could vote to be shut down for a day, effectively going on strike.
Developers would enjoy the privileges of membership in the union, because their support calls would get answered and any individual threats of takedown or lawsuit would face the consequences of union attention.
Apps could even be certified as "Fair Made" so users could find "sustainable" apps.
Maybe there would be other functions, to provide independent news on the state of development and any shenanigans like attacks on net neutrality or abuse of the patent system.
All that is missing is support from companies that have achieved success, such as Rovio (of Angry Birds fame), PopCap and humblebundle.com.
An organization like this will be protected under the National Labor Relations Act:
http://en.wikipedia.org/wiki/National_Labor_Relations_Act
I did a quick search on developer unions here and all I found was this:
http://news.ycombinator.com/item?id=2278166
So my question is, where do we go from here?