If you’re interested in building large-scale Flex apps and not sure where to start, Brian Riggs has a nice article on Architecting a Flex App, where he gives a good overview and introduction to structuring an application using the Model-View-Controller pattern, as implemented in the Cairngorm framework.
Brian is one of two (at least so far, maybe there are more) authors writing on the Adobe Kiwi Project blog. The Kiwi Project (as stated in the tagline of the site) is a project which is working on creating “Read/Write Web Components for Flex.” I’m not sure of the details of what that means, but parts of it can be seen in the open-source NoteTag note-taking application which Brian uses as his example in this and other articles, and which is available on the Adobe Labs site.
Darrick Brown, who also writes on the site, has put up a couple of nice articles on ActionScript 3 from a C/C++ perspective. I don’t know either of those languages, although I work a lot in C# so I understood all his examples just fine. In any case he’s got what I think are some nice examples and thoughts for someone coming from any modern language.