Just spent some time watching a presentation by Ali Mills and Luke Bayes which reviews the available Flex Frameworks. If you are looking for an up to date summary of the available Flex Frameworks you really can't go past this presentation. If I were to summarise their findings it's that most Frameworks are still too immature. The two exceptions they identified were Cairngorm and PureMVC. They felt that Cairngorm was too tightly coupled to it's Singleton's and that this made unit testing very problematic. In contrast they were very taken with PureMVC. They felt it was a more elegant solution despite it's use of Singleton's. If you are interested in Flex and Frameworks you should watch this very informative presentation.
Update : a post presentation debrief (with link to presentation) is available. The most interesting addition here for me is the following quote :
"Doug McCune brought up a really great point about Cairngorm. He mentioned that if you’re a contractor that is working on other people’s projects and going from client to client, the only framework that you must know is Cairngorm. He made the point that it’s the most likely Application Framework that you’ll encounter within the enterprise. We both agreed with that point, but added that you should investigate PureMVC as the very next step."