Just a note on the subject of the thread.
The stuff we're discussing is not an rdtool killer: it's an interface
documentation system. rdtool is different: it's a more generic markup
language. Sometimes it appears in Ruby source files, but it also
appears standalone and as markup for web pages. rdtool really is not
at its best doing per-method documentation, and RubyDoc (or whatever)
can't effectively be used to write books. So one doesn't kill the
other: they just specialize into their own niches.
Dave