For a clear explanation of why XML is being championed as the greatest thing for publishing since Gutenberg (version 1.0), Book Brunch has a great explanation of both the Start with XML project and a gently expansive view of Move Towards Semantic.
As the book evolves past the once definitive borders of covers and bindings and becomes content without a container, standards for book publishing need to also be extensible to allow for ease of compatibility.
George Lossius of Publishing Technology makes this point far more eloquently that I:
We have to view this opportunity in terms of Semantics as well as Syntactics. This is why, although I support fully the move to extending the use of XML, I feel that if we limit our focus to an XML standard Schema for publishing alone, we are forgoing an opportunity to embrace our broader presence in the media space. I am suggesting that an XML process, which provides a Syntax for communication, needs to be allied to RDF (Resource Description Framework) which provides a Semantic description for communication on a web-wide basis.
Thanks to Tools of Change for the link.