This is a minimal implementation, as DocBook lacks a serious way of encoding all of this. Maybe a <formalpara> could do the trick, but I'd need to find a way to shoehorn a title through the styles (i.e. a first complete tag):
Theorem: Bla bla
<formalpara>
<title>Theorem</title>
<para>Bla bla</para>
</formalpara>
This would also only be a solution for single-paragraph things, as formalpara only allows one paragraph. Or a sidebar, but it's semantically very remote.
Includes: semantic markup (sorry about noun: Additional.lyx uses it to mark menus; there is something better in DocBook, but it looks like the LaTeX equivalent is really for person names), boxes, info layouts.
