redcarpet.rb

# config/initializers/redcarpet.rb

class ActionView::Template
  class Redcarpet < Handler
    include Handlers::Compilable

    def compile template
      ::Redcarpet.new(template.source).to_html.inspect
    end
  end

  register_template_handler :markdown, Redcarpet
end

Updated for rails 3.2.x

module MarkdownTemplateHandler
  def self.call(template)
    "Redcarpet::Markdown.new(Redcarpet::Render::HTML, :autolink => true, :space_after_headers => true).render(#{template.source.inspect}).html_safe"
  end
end
ActionView::Template.register_template_handler(:md, MarkdownTemplateHandler)