Graeme,
Yes, the plan is to eventually address other output formats such as PDF as well. There are just a lot of other things to solve first, so we'd be getting ahead of ourselves if we tried to start there.
Producing PDFs consistently and with full fidelity is always going to be a challenge since PDF is a such a constrained and esoteric format. I've come to accept more recently that the most direct and capable path to get there is to go through the browser engine (web to PDF). The reason is that the web provides a tremendously strongly layout engine which can then be readily distilled into the fixed, page-oriented medium of a PDF. We're currently experimenting with this approach in the Asciidoctor project.
Having said that, the main focus of the specification project, at least initially, is only the AsciiDoc language, not the output formats. The whole idea behind AsciiDoc is to give you the power and control to produce the publishable format you want to create. So while there will be some stipulations about what must be produced, we will never go so far as to dictate the style. That variation you talk about is one of the strengths, not weaknesses.
Best Regards,
-Dan