Markdown — see Wikipedia — is becoming really well supported, with loads of tools to convert to (and in some cases from) HTML, PDF, LaTeX, DocBook, groff etc etc. This makes it a great choice as a standard doc format, much easier to write than an XML dialect, and also a good choice as a simple wiki format.
You can see some of the available engines in action at
or have a look at the converter at http://johnmacfarlane.net/pandoc/try
The PHP implementation is one of the most solid. Is there any interest in adding an option to use the portable Markdown syntax instead of the (probably more powerful) Wikidot syntax?