Wesner has noticed that XAML can be used pretty broadly as a document format "If application developers start writing document-based applications that follow the Avalon model of deriving new classes from existing elements, or even simply reusing existing elements, and turning on some of Avalon's designer services, XAML could quickly become one of the most common file formats."

In fact, if you look at the container support in Longhorn (System.IO.CompoundFile, if I remember right) there is more than just XAML in here. With Longhorn we are doubling down on OLE compound files to have a good way to have complex documents with rich metadata.