We all love Java code but in my opinion building GUIs using code only, is a hard task. A hard task to build, a hard task to read and a hard task to maintain.
Currently I am working with Flex 4.x and it feels like defining components in declarative way seems to make things easier/faster for the programmer. Take also for example JSF. I love the way I build pages using JSF with XML and using the expression language and everything. I also believe that it would also be easier to create an application/node designer easier in this way. Another thing to consider would be states which were available in the Netbeans JavaFX designer.
I know it's early to expect something like this from Oracle but I really believe that it would also help Flex developers to switch to JavaFX easier. If I am not mistaken Silverlight also uses XML.
I see it implemented in a similar manner as the JSP XML syntax compilers are implemented. XML -> Java Source File -> Java bytecode.
If Oracle is considering something like this which would also be open source I would certainly participate or if you feel like starting an open source project somewhere sometime (soon) let me know.
Will that be a community produced extension, similar to the way that fxscript has spun out to a community effort?
I would favor something community based so that extensions area easier. Perhaps just implementing the XAML specification would be sufficient. Infusing the XAML specification with some dependency injection thinking would make it a differentiator in the marketplace.