The Flex client works exactly like the Flash 9 client and shares an almost identical codebase. 99% of the framework is the same.
Today I'm releasing The GAE SWF Project, a resource of Flash and Flex-related knowledge specifically aimed at getting you up and running quickly with Google App Engine. If you want to skip all the details and start playing with it, skip to the Getting Started section.
I just read a blog post and watched a screencast about a new Flash framework called Gaia and I like most of what I saw.
In the comment, Pondus states: