The standard Apache module for Markdown is now Hoedown, which supports lots of extensions, is written in no-dependency C, and is essentially bulletproof. It also separates the parser from the renderer, which is important if you want to index various text pieces. There is a CPAN Text::Markdown::Hoedown module which compiles and passes tests on my box; we'll see how easy it is to use (not much documentation, but that can be fixed with a pull request...)
In poking around and reading the documentation for Markdent, I found a couple of interesting proposals for Markdown extensions from Dave Wheeler: definition lists and better tables with multiline content.
So what I really want is a Hoedown-based parser generator that can add extensions at will. (Maybe later...) Because honestly, what this is all about is different ways to use simple 2-dimensional, as opposed to 1-dimensional, arrangements of punctuation to delimit different items in text in as general a way as possible, build data structures based on that text, then render presentations based on those data structures. Each level of that process is interesting in and of itself.