Very good news from the Adobe labs, the Flex Component Kit allow us to create Flex component with Flash CS3:
The Flex Component Kit for Flash CS3 allows you to create interactive, animated content in Flash, and use it in Flex as a Flex component. This is prerelease software.
Use the Flex Component Kit when you want to use interactive or animated Flash content in Flex. The Flex Component Kit converts your Flash content into Flex components. These components work with Flex features such as view states and transitions, skinning, and tool tips. The content can be used as a child of a Flex container, or as a skin for any Flex component.