The latest version of Flex is now in beta and you can download it from Adobe labs for free.  Instead of calling it Flex Builder, they are now calling it Flash Builder.  The framework is still called Flex.  Anyway, download it and be sure to watch the videos that tell you what is new in this version.

Download: http://labs.adobe.com/technologies/flashbuilder4/

Video Tutorials: http://labs.adobe.com/technologies/flash/videos/