I moved mine from Tumblr to GitHub recently and used a migration script to grab all the posts and turn them into Markdown files. There's a script that works with Posterous too: https://github.com/mojombo/jekyll/wiki/blog-migrations
edit: I've just realised I don't need the code to be merged into jekyll in order to use it (facepalm).
edit2: ok, maybe not as straightforward as I thought. I'm not really a coder so it takes me longer to understand what scripts are doing (and then tweak them so they work for me).