I’m not sure if it’s in the context of this project, but I’d find replacing double-hyphen with a dash like TeX does very useful. I think that especially in blogs, which have some “literate” quality, hyphens should not be used in place of dashes, since they are very different characters – one combines and one separates. I find the TeX solution very convenient, and I think that it is true to the Markdown spirit, in that it keeps the ASCII source readable by humans.
It can be learned from http://en.wikipedia.org/wiki/Dash#Electronic_usage that there’s generally no easy way to enter a unicode dash. I think that making dashes easy would somewhat improve the quality of Markdown documents.