Teresa Mulvihill has written an article on DocBook vs. DITA at: http://www.dclab.com/dita_docbook.asp.
It's a well thought-out article, but I'd like to make a few clarifications:
As you may know, Jim Earley and I have presented on this topic and our Doc Standards Interoperability Framework at several conferences, and still hope to form an OASIS TC on document standards interoperability.
In the article, Teresa states:
"DocBook is hierarchical by nature, and must be developed to allow for single-sourced content. DocBook has a fixed element and attribute set."
I've successfully used and recommended single-sourcing approaches with DocBook, without additional development. It's quite easy to set up a book or article and use XIncludes or even file entity references to pull in content from a common pool of content structures (usually section or chapter).
It's also fairly easy to extend the elements and attributes in DocBook. This has been made even easier with DocBook v5.0 and RelaxNG. In fact, the DocBook Subcommittee for Publishers that I chair, has helped organize the source patterns for DocBook v5 with a modular approach, enabling easier customizations to be created. Our subcommittee has created a customization geared specifically to publishers, without all of the technical blocks and inlines in full DocBook. This significantly reduces the tag set for folks that do not produce software or technical documentation to use the DocBook standard for general publishing!
DocBook can generate more than PDF, HTML and HTMLHelp. The docbook-xsl-1.73.2 stylesheet distribution supports: html, htmlhelp, javahelp, manpages, xhtml, Word roundtrip, slides and websites!
I would also argue that DocBook can be used on very high volume documentation projects, as well as small and medium projects. Just ask Sun, HP, various Linux distros, and more listed here: http://wiki.docbook.org/topic/WhoUsesDocBook
You might also find Norm's blog on DocBook vs. DITA interesting: http://norman.walsh.name/2005/10/21/dita
We are also working on some exciting developments for the Interoperability Framework, so stay tuned!