Is the Windows 2009/Avalon XAML Format Extensible or Versionable?

Hello,

  I just stumbled over an article by Dare Obasanjo - Microsoft's .NET
XML tech lead - titled "Designing Extensible, Versionable XML Formats".

   Dare writes:

    An XML vocabulary should be designed in such a way that the
applications that process it do not break when it is inevitably changed.

    ...

     Why XML formats should be designed to be extensible.

  The primary benefit of allowing extensibility in a format is that it
enables a format to evolve without requiring central control of the
format. A secondary benefit is that it allows the format to stay
focused and simple by pushing specialized or niche-use cases and
complex solutions into optionally supported extensions.    

   More @ http://www.xml.com/pub/a/2004/07/21/design.html
     and http://www.xml.com/lpt/a/2004/07/21/design.html

   What's your take on it?
   Do you think the Windows 2009/Avalon XAML format is extensible or
versionable or do you think that the Windows 2009/Avalon XAML format
is bound so tightly to the Windows 2009/Avalon API that it's not
really a free-standing extensible and versionable XML format but just
a XML dump. Any thoughts? Any comments?