added markitup viewhelpers and force output to a string

gistfile1.rb

module FormtasticExtensions
  module Formtastic
    module MarkItUpEditor
      include MarkItUp::ViewHelpers

      def self.included(base)
        base.class_eval do
          @mark_it_up_dependencies_included = false
        end
      end

      protected

      def mark_it_up_input(method, options = {})
        dom_id = "#{@object_name}_#{method}"
        output = ''
        output << text_input(method, options)
        output << mark_it_up(dom_id, options[:mark_it_up], @mark_it_up_dependencies_included)
      end
    end
  end
end

if Object.const_defined?("Formtastic")  
  Formtastic::SemanticFormBuilder.send(:include, FormtasticExtensions::Formtastic::MarkItUpEditor)
end

Adding output='' prevents the output being escaped by formtastic