export_comments.rb

# rake task to export comments from typo to disqus

require 'disqus'
require 'disqus/api'
require 'disqus/author'
require 'disqus/forum'
require 'disqus/post'
require 'disqus/thread'

namespace :export do
  desc "Load Blog comments to Disqus"
  task :comments => :environment do
    Disqus::defaults[:account] = 'xxxx' # your user account
    Disqus::defaults[:api_key] = 'xxxx' # your api key http://disqus.com/api/get_my_key/
    site_url = "http://yourwebsite.com/articles/"
    forum = Disqus::Forum.list.first
    Content.find(:all, :conditions => "type = 'Article'").each do |post|
      
      if post.comments && post.comments.size > 0
        # create thread
        permalink = site_url + post[:published_at].strftime('%Y/%m/%d/') + post.permalink
        thread = Disqus::Api.thread_by_identifier(
                  :forum_api_key => forum.key, 
                  :identifier    => post.title, 
                  :title         => post.title)

        begin
          Disqus::Api.update_thread(
              :forum_api_key => forum.key, 
              :thread_id     => thread["message"]["thread"]['id'], 
              :url           => permalink,
              :slug          => post.permalink,
              :title         => post.title,
              :allow_comments => 1) # if you want to close comments on old posts
        rescue => e
          puts permalink
          puts e
        end
        post.comments.each do |comment|
          # insert comments
          next if comment[:state] =~ /Spam/
          # FIXME: better way to manage author ?
          Disqus::Api.create_post(
                   :forum_api_key => forum.key,
                   :thread_id     => thread["message"]["thread"]['id'],
                   :message       => comment.body,
                   :author_name   => comment.author,
                   :author_email  => comment.email, 
                   :created_at => comment.created_at.strftime("%Y-%m-%dT%H:%M")               
              )
        end
      end
    end
  end # export:comments
end

Nice one, I'd been wondering how you might do this!