The Digital Editions blog is reporting that the latest release of the DocBook XSL (1.74.0) now supports output to the ePub format. It was only recently that the tei2epub converter was announced so it is great to have a converter from DocBook to the ePub format.
DocBook is a very mature XML master format that although is used mainly for technical documents, can also be used for simpler documents and prose like books, which could make it a useful master format for any public domain eBook projects.
This release of the DocBook XSL is an experimental release and is being made available for testing purposes. Any developers out there should certainly check it out and if you find any issues, report them back to the DocBook project.