According to this Ticket, using form_tag with a block in a helper-method is broken in rails 2.1.

Here is a, not so elegant but working, solution:

   1  def render_form_tag_in_helper( _erbout, url, *args )
   2    form_tag( url ) do
   3      concat( submit_tag("button"), binding )
   4    end
   5  end

We have to pass the _erbout-variable to the helper-method in order to make the form_tag-method work. And to add elements to the form (submit_buttons, text_fields, etc), we have to call concat (to add the elements to the _erbout variable) with the current binding.