One can configure Flex facet for the module (in Project Settings dialog there is 'new' action on particular module node) giving the location of the Flex SDK. This allows better complete / navigation since IDEA will index Flex SDK sources.

Why use a facet for Flex support? Why not make it a module?

Making it a facet implies that it should/will be used together with Java. I think many people, including myself, develop Flex as more "module style". That is, one language per module.
GWT as facet makes, as does Hibernate, Spring and so on. But I would consider AS and MXML quite orthogonal to Java development.

Also making it a separate module would bring other benefits, such as enabling assignment of a dedicated Flex SDK type, much like the Ruby module has today.

I second Carl's note; it makes no sense to tie Flex projects to Java projects. Once Idea becomes capable of supporting mxml and as with Flex SDKs, I should be able to create Flex only projects, otherwise organizing projects becomes silly: it becomes a must for me to pick some Java project to attach my Flex code to.

There is Flex module type in Diana

Maxim, so that means that I'll be able to create a project with only Flex modules? If that happens, if it works well then you JetBrains will be getting Flex developers defecting from Adobe Flexbuilder to Idea, that's money, you know...

Yes (IDEA-41803)