For Flex, the idea of open source documentation encompasses the four main Adobe-based sites that developers use to learn about Flex:
The Flex 3 GSE (http://learn.adobe.com/wiki/display/Flex/Getting+Started) is built on a wiki where it's easy to update pages and relatively easy to add them. This site is aimed at developers who are new to Flex it's a big improvement over what we offered in Flex 2. That said, there's room for improvement and we're looking for developers to add, modify and monitor content.
For LiveDocs pages, we'll use a system similar to what contributors do in open source Flex. That is:
Many developers are looking for how-to topics, and the Flex Cookbook is perfect for this type of information. We'll also try to add links to Cookbook entries from related pages in LiveDocs. For more information, see http://www.adobe.com/cfusion/communityengine/index.cfm?event=homepage&productId=2
We also classify contributions to the Flex Developer Center as contributions to open-source documentation.
If you're interested in contributing to this effort, contact Randy Nielsen (ranielse AT adobe DOT com)