The Free Software Foundation intends to support DocBook as an interchange format. Texinfo has enough structure to make reasonably good automatic conversion possible (human editing is still needed afterwards, but not much of it), and the 4.x versions of makeinfo feature a --docbook switch that generates DocBook. More at the makeinfo project page.

A POD::DocBook module translates Plain Old Documentation markup to DocBook. It claims to translate every POD tag except the L<> italic tag. The man page also says “Nested =over/=back lists are not supported within DocBook”, but notes that the module has been heavily tested.

A project called TeX4ht can, according to the author of PassiveTeX, generate DocBook from LaTeX.

These are generally considered the biggest and nastiest conversion problems. And indeed, the basic troff(1) markup is at too low a presentation level for automatic conversion tools to do much of any good. However, the gloom in the picture lightens significantly if we consider translation from sources of documents written in macro packages like man(7). These have enough structural features for automatic translation to get some traction.

I wrote a tool to do troff-to-DocBook myself, because I couldn't find anything else that did a tolerable job of it. It's called doclifter. It will translate to either SGML or XML DocBook from man(7), mdoc(7), ms(7), or me(7) macros. See the documentation for details.