Macromedia gives a preview of their upcoming development tool Flex. It's a server-side application that runs alongside J2EE servers which generates Flash dynamically from XML descriptions. Unlike XAML, it's actually much more similar to XUL. It even has HBox and VBox tags. No mention of whether it supports the flex attribute as suggested by the name.
It's entirely server side and generates Flash to be sent to the client. Since it's Java-based, it does have support for interacting with server Java objects and such.
There's little documentation at the moment. I could only find a white paper and an introduction, so I'm not sure how sophisticated Flex really is.