XML based generation of Rich and Native UI's is gathering momentum, it might also be a point to understand the complimentary relationship that exists between XForms and these XML based GUI generators.
BTW - Here is a great XForms presentation that helps aids in the contextualization of my prior comments.
The actual Macromedia MXML (Flex) review by Jon Udell follows:
After a decade of web-style development, I'm sold on the idea of using markup languages to describe the layouts of user interfaces and to coordinate the event-driven code that interconnects widgets and binds them to data. The original expression of that model was HTML and JavaScript, but variations have flourished. Mozilla-based applications have been using XUL (XML User Interface Language) for years. The Laszlo Presentation Server uses a description language called LZX. Microsoft has previewed XAML (Extensible Application Markup Language) for Longhorn.
Now comes MXML (Macromedia Flex Markup Language), the latest development in Macromedia's ongoing quest to reposition the near-ubiquitous Flash player as a general-purpose presentation engine for rich Internet applications. With XML markup at its core, Flex is inherently IDE- friendly, and Macromedia has two IDE initiatives underway. One, code-named Brady, builds on Dreamweaver MX. The other, code-named Partridge, leverages Eclipse.
Full Review: http://www.infoworld.com/article/04/03/29/13TCflex_1.html
Also see XML for UI Languages: http://xml.coverpages.org/userInterfaceXML.html
Nothing stops any of the engines mentioned above (proprietary user interfaces as per the diagram below)