Source code for: http://baylorrae.com/how-i-use-markdown-with-wordpress/

functions.php

<?php
function markdown_filter($data, $post) {

  // change this to match your heroku app url
  $ch = curl_init('http://radiant-stone-4578.heroku.com/parse');

  curl_setopt_array($ch, array(
    // don't print the result when done
    CURLOPT_RETURNTRANSFER => true,

    // use a post request and pass on the content
    CURLOPT_POST => true,
    CURLOPT_POSTFIELDS => 'markdown=' . urlencode($data['post_content'])
  ));

  // grab the parsed content
  $parsed = curl_exec($ch);

  curl_close($ch);

  // back up the original markdown
  $data['post_content_filtered'] = $data['post_content'];

  // save the new parsed html
  $data['post_content'] = $parsed;

  return $data;
}

function get_markdown($content, $id) {
  $post = get_post($id);

  // make sure the post has content saved in post_content_filtered 
  if( $post && !empty($post->post_content_filtered) )
    $content = $post->post_content_filtered;

  return $content;
}

/**
 * I'm not entirely sure why this is done
 * it was used in the "Markdown on Save" plugin
 * and I kept it in case it was needed
 * 
 * @author Mark Jaquith
 */
function get_markdown_filtered($content, $id) {
  return $content;
}

// on post save/update
add_filter( 'wp_insert_post_data' , 'markdown_filter' , '99', 2 );

// when editing a post
add_filter('edit_post_content', 'get_markdown', '99', 2);

// when editing post_content_filtered
add_filter('edit_post_content_filtered', 'get_markdown_filtered', '99', 2);
?>

parser.rb

require 'rubygems'
require "sinatra"
require "redcarpet"

get "/" do
  "I'm alive, I'm alive, I'm ... why are you not excited?"
end

post "/parse" do
  markdown = Redcarpet::Markdown.new(Redcarpet::Render::HTML,
    :autolink => true, :space_after_headers => true, :fenced_code_blocks => true, :no_intra_emphasis => true)
    
  markdown.render(params[:markdown])
end