I'm super excited that JGM (of Pandoc) is heading this and has some cooperation from Github and Reddit (both the largest users of Markdown that I'm familiar with). This is something I've hoped to happen for a long time.
Given that, the lack of a formal definition is even more surprising: why did they not spec it in Haskell? Formal, unambiguous, and as a side benefit, executable.
StackOverflow? (who're also on board via Atwood)
That said I'm sure he has valuable experience with Markdown and is almost certainly using it again in his latest venture.