Note there is a "compact syntax" version but it is not an ISO standard. The notation is non XML, but can be converted to RelaxNg without information loss.
There was a concern registered about the acceptability of RelaxNg in a government environment. That was before the ISO adoption and I am unsure what is the situation and outlook at end 2004.
From the charter at http://www.oasis-open.org/committees/relax-ng/ :
The purpose of this committee is to create a specification for a schema language for the ExtensibleMarkupLanguage based on TREX and RELAX. The key features of RELAX NG are that it is simple, easy to learn, uses XML syntax, does not change the information set of an XML document, supports XML namespaces, treats attributes uniformly with elements so far as possible, has unrestricted support for unordered content, has unrestricted support for mixed content, has a solid theoretical basis, and can partner with a separate datatyping language (such W3C XmlSchema Datatypes).
Links for RelaxNg ancestors: