Due to the lack of a convenient way to export the Markdown from a Ghost blog, I've written a nodejs module to do exactly that! It's called
ghost-export and you can install it like this:
$ npm install -g ghost-export
Then, you can use it like so:
$ ghost-export /path/to/ghost/app /path/to/output
You'll end up with a directory full of Markdown files from your blog. Simple!
For the source and more information, check out ghost-export on Github!