Given a WP XML export, this script will generate the necessary 301 redirect rules for all posts for Apache or Nginx.

octopress_redirects.rb

require 'date'
require 'rexml/document'

include REXML

doc = Document.new(File.new(ARGV[0]))
format = ARGV[1] || "apache"

formats = {
  :apache => "RewriteRule ^%s$ /%s [R=301,L]",
  :nginx => "rewrite ^%s /%s permanent;"
}

File.open("redirects.txt", "w") do |f|
  doc.elements.each("rss/channel/item[wp:status = 'publish' and wp:post_type = 'post']") do |e|
    post = e.elements
    date = DateTime.parse(post['wp:post_date'].text)
    slug = post['wp:post_name'].text

    post['link'].text =~ %r{.*(/archives/\d+)}
    old_url = $1
    new_url = "blog/#{date.strftime("%Y/%m/%d")}/#{slug}/"

    f.puts formats[format.intern] % [old_url, new_url]
  end
end