Adobe announces plans to open-source Flex under the Mozilla Public License (MPL). Developers will soon be able to freely download, extend, and contribute to the source code for the Flex SDK, including the Flex compiler, command-line debugger, and application framework.
You can find more info on Adobe Labs

The Adobe to open-source Flex by David Gamez, unless otherwise expressly stated, is licensed under a Creative Commons Attribution-Noncommercial 2.5 Mexico License.