This is very good news! Flex builder and flex data services are not being open sourced at this time... bummer. :)
"Adobe announced strategic plans to move the development of Flex to an open source model."
"This includes not only the source to the ActionScript components from the Flex SDK, which have been available in source code form with the SDK since Flex 2 was released, but also includes the Java source code for the ActionScript and MXML compilers, the ActionScript debugger and the core ActionScript libraries from the SDK."