Most people still hack DocBook tags by hand using either vi or emacs. There's an Nxml mode that ships with Emacs and is automatically invoked when the editor recognizes an XMl document. It has become pretty good; while it doesn't give GUI presentation, it does use its knowledge of XML to highlight out-of-balance tags. Some alternative are summarized at the Emacs CategoryXML page.
There have been a number of attempts at GUI editors for DocBook, often with the aim of being general editors for any markup with an XML or SGML schema. EuroMath, MLView, Conglomerate, ThotBook are among them. Such projects tent to stall out in alpha stage; designing a decent UI for this task is extemely difficult.
Some attempts that have made it to production stage (if only barely, in many cases) can be found at the DocBook Authoring Tools page. I have not tried using any of these.