Since Flex 3 was released at the end February 2008 developers have been able to see progress within the Flex SDK by monitoring the commit list and downloading nightly or stable builds from the Flex SDK Open Source site. Flex SDK 3.1 is what the team considers a milestone build, a certification of all of that work and a roll-up into a build that is recommended for all users. Flex Builder 3.0.1 is an update that includes the Flex 3.1 SDK along with numerous bug fixes and certified versions of the professional data visualization and testing components.
The Flex 4 SDK, code named Gumbo, is up for download now. This is an exciting time for Flex, it's now 2 full versions beyond Flex 2 which was really the first version of Flex that gained mainstream adoption
"Additionally, Linux developers can create, test and deploy rich Internet applications (RIAs) on the Linux platform using the free Adobe Flex 2 SDK, Adobe Flash Player 9 and the free Flex Data Services 2 Express."