Flex Builder 4 (next version) will be called Flash Builder
Adobe has just announced that the next version of Flex Builder will be called Flash Builder .. the framework will still be called Flex
I think although the change is confusing, in the long run it adds clarity ... here's why ...
1. Flash Builder (formerly Flex Builder) is an IDE to write .SWF (Flash Player) files .... no matter what application framework you use, it could be pure AS3 code (i.e your own framework) , Flex framework or anything else .. there are many other framework options (although Flex is the most mature) ... Flash Builder can be used to build in any of those frameworks.
2. Flex is a framework of classes that solve several everyday application development problems when building apps that can run in the Flash Player ... you can use any tool to write your own classes that use flex framework classes (a flex application) ... Flash Builder (previously Flex Builder) or FDT or a text editor like TextMate.