gistfile1.rbx

module ActionView
  module Helpers
    class FormBuilder
      module FormBuilderExtension
        def hint(method, text)
          errors = object.errors.on(method)
          hint_message = "<span class='hint'>#{text}</span>"
        
          if errors.blank?
            hint_message
          else
            error_message_on(method, "<span class='explanation'>", "</span>#{hint_message}", 'errorHint')
          end
        end
      end
      
      include FormBuilderExtension
    end
  end
end

gistfile2.css

.errorHint .hint { display: none; }
.errorHint:hover .explanation { display: none; }
.errorHint:hover .hint { display: block; }