I hope development of this spec will be perpetual, similar to the WHATWG's HTML(5), if it happens at all. I've been dealing with markdown edge cases for more than a year now and I'm always finding new things I missed. It's really amazing how markdown is just a giant string of special cases and exceptions (this is probably due to the way it was initially written). The post by Jeff Atwood lead me to believe it's supposed to be geared more toward features as opposed to exact specifications. (I could be wrong about this).
Although I agree Gruber seemed kind of impolite and dismissive here, I think we should take things slow, and not rush into making a "spec" right away. The closest thing there has ever been to a markdown spec was Gruber's original test suite (on top of the markdown docs). It is actually kind of hard to track down since you can't find it on his website (I don't think I saw it mentioned in these articles either). My test suite is based on a fork of it ( https://github.com/chjj/marked/tree/master/test/tests ). That test suite covers no where near enough to demonstrate everything. I have a feeling this spec would take a long time to complete, especially if it's including more compex features like GFM tables and whatnot. There will also probably be a ton of arguing over which features should be included.