In KDE3 Kate had a plugin called "XML Completion Plugin". It was able to list XML elements, attributes and attribute values and entities in a completion popup menu depending on the currently active DTD. For instance, it was able to complete all your KDE/docbook tags and, thus, was one of the best tools for writing KDE documentation.
Unfortunately, this plugin has not been ported to KDE4, yet. So this is a request for developers: Please someone pick it up and make it available for Kate in KDE4.
The location in svn is: trunk/KDE/kdesdk/kate/plugins/xmltools
Just enable this folder in the kate/plugins/CMakeLists.txt file and start porting it. The code completion interface changed quite a lot, but there are other plugins and good api documentation (KTextEditor interfaces, Kate Application interfaces) where you can have a look at how things work.
Having a working XML Completion plugin for KDE4.5 would be awesome.