(shameless plug) I wrapped Pandoc[1] in a web service and added on nice PDF exports and called it Docverter[2]. It will convert basically anything plain-text, including Markdown, into almost anything else plaintext, HTML, RTF or Docx. I also added rich PDF exports that go through a HTML intermediary.

If this gains some traction I'm sure I'll be adding support for it at some point.

[1]: a wonderful almost-everything-to-everything text converter http://johnmacfarlane.net/pandoc/

[2]: http://www.docverter.com