I just saw
Why re-invent the wheel?, posted 20 Sep 2002 by kevindumpscore (G.R. Article: Interop in the Bazaar)
At OSCOM (www.oscom.org) we created SlideML exactly because there are several XML presentation languages around and everyone thinks his/hers is the best. With SlideML we didn't reinvent only a language to write OSCOM presentations, but also a translator with which you can go to Docbook Slides, but also to OPML, to SVG, whatever.
IMHO SlideML is a new presentation mark up language that isn't devolped on scratch but integrates the best from the already existing xml presentation formats. It is also a true interop format (which none of the other presentation languages is today). It is lightweight (XHTML) and powerfull (full use of the neweste XML trends: Namespaces, XInclude, Dublin Core) at the same time. It is extendable, but also simple.
As for docbook
I for myself like Docbook quite a lot (we use a subset in our Bitflux CMS and we did our presentation at OSCOM 1 in Docbook Slides with our CMS) and I am very glad that XHTML 2.0. comes nearer to Docbook than any X/HTML before. But for some people Docbook is to heavy and they would prefer to use XHTML to write their presentation. So SlideML is some compromise, but a compromise with a lot of benefits for everyone (the Docbook Slides enthousiasts, as well as the AxKit, the OPML, the SVG,... enthousiasts.
I hope that clarifies some misunderstandings that I thought reading in Kevin's comment.