The OASIS OpenDocument Format 1.2 introduces RDFa and RDF/XML support.
An ODF document is a ZIP package containing multiple resources, typically including at least a content.xml (the contents and structure of a document) and a styles.xml (the style definitions). ODF 1.2 defines a subset of RDFa for use in annotating elements in content.xml and styles.xml: @about, @property, @datatype and @content. RDF/XML is also supported, where this metadata is stored as a separate XML doc in the ZIP file.
The core specification of this is in ODF 1.2 Part 1, section 4.2 and in Chapter 6 of ODF 1.3 Part 3.
The blog KOffice & RDF: Who, What, When, Where? gives some background on how RDF is used in KOffice, e.g. to store FOAF and iCal info in ODF documents..
OpenOffice.org exposes RDF programmatically and OpenOffice plugins can be implemented to support specific uses cases.