To import your posts from Tumblr, run:

$ ruby -rubygems -e 'require "jekyll-import";
"url" => "http://myblog.tumblr.com",
"format" => "html", # or "md"
"grab_images" => false, # whether to download images as well.
"add_highlights" => false, # whether to wrap code blocks (indented 4 spaces) in a Liquid "highlight" tag
"rewrite_urls" => false # whether to write pages that redirect from the old Tumblr paths to the new Jekyll paths

The only required field is url. The other fields default to their above values.