I converted this site over from Tumblr to Octopress. Since I had hundreds of writings that I wanted to preserve, manual migration was not an option, I needed something that would wholesale export my content from Tumblr and prepare it in a format that would be suitable for publishing with Octopress.
I did not find such option, so I just wrote a small migrator script. You give it some data about your Tumblr site, and it exports all your content in a format that can be easily consumed in Octopress.
There were a bunch of other exporters available, but they seemed to mostly get the already published site content, which did not work for me: I wanted the raw Markdown.
Optionally, the script also downloads the images that you posted in Tumblr using its web-publishing URL, and replaces the URLs in post sources, so you can completely remove your Tumblr dependency.