How can I automatically render partials using markdown in Rails 3?

Turns out, the Right Way ™ to do this is using ActionView::Template.register_template_handler:

lib/markdown_handler.rb:

require 'rdiscount'

module MarkdownHandler
  def self.erb
    @erb ||= ActionView::Template.registered_template_handler(:erb)
  end

  def self.call(template)
    compiled_source = erb.call(template)
    "RDiscount.new(begin;#{compiled_source};end).to_html"
  end
end

ActionView::Template.register_template_handler :md, MarkdownHandler

If you require 'markdown_handler' in your config/application.rb (or an initializer), then any view or partial can be rendered as Markdown with ERb interpolation using the extension .html.md:

app/views/home/index.html.md:

My awesome view
===============

Look, I can **use** <%= @language %>!

app/controllers/home_controller.rb:

class HomeController < ApplicationController
  def index
    @language = "Markdown"
  end
end