The Macromedia Flex Developer Center is showcasing some example applications of a new approach to interface design for rich internet applications. Flex is a server-based system that leverages the ubiquitous flash client to enable developers to build dynamic data-driven online applications using Macromedia's own XML variant (mXML).
The tool seems best suited to information dashboards, shopping systems, catalogues and specific interfaces that could benefit from the richness and data processing that Flash and XML together can provide. It sits on top of any J2EE server and Macromedia have said they will later release a .NET version.
At $12k per double CPU server and with something of a learning curve, it had better be good!
UPDATE: Good piece from Jon Udell comparing Flex with XAML.
I am a UI designer/coder and have been learning Flex for the past few weeks. It is very powerful, but is very different than what DHTML coders might be used to. You have to know a lot more about Object Oriented programming and the API is pretty big. The great part is that if you work with J2EE guys they will get it right away (unlike working with Flash), which makes developing apps a lot easier!