While the CVS version of the maven sdocbook plugin already does it, if you want to generate xhtml instead of html with the latest 1.4.1 release, you only need to set the following properties (e.g. to project.properties).

maven.sdocbook.html.xsl=${maven.sdocbook.stylesheets.dir}/xhtml/docbook.xsl
maven.sdocbook.htmlchunk.xsl=${maven.sdocbook.stylesheets.dir}/xhtml/chunk.xsl