InstantRailsでtextilize使うなら

gem install redcloth

が必要なようね。。。

でくくられたくないときは

<% textilize_without_paragraph @sometext %>

みたいなのでよいようだ

調べてみたらactionpack(versionの指定もいるのかな)の下の

lib/action_view/helpers/text_helper.rb

というファイルなようだったので

        def textilize(text)
          if text.blank?
            ""
          else
            textilized = RedCloth.new(text, [ :hard_breaks ])
            textilized.hard_breaks = true if textilized.respond_to?("hard_breaks=")
            textilized.to_html
          end
        end

        # Returns the text with all the Textile codes turned into HTML-tags, but without the regular bounding <p> tag.
        # <i>This method is only available if RedCloth can be required</i>.
        def textilize_without_paragraph(text)
          textiled = textilize(text)
          if textiled[0..2] == "<p>" then textiled = textiled[3..-1] end
          if textiled[-4..-1] == "</p>" then textiled = textiled[0..-5] end
          return textiled
        end

こういう実装だった