gistfile1.rbx

module ActionView
  module Helpers
    class FormBuilder
      def submits(create_value = "Create", update_value = "Update", options = {})
        value = @object.new_record? ? create_value : update_value
        submit(value, options)
      end
    end
  end
end

gistfile2.txt

/ A HAML form partial
- form_for current_object do |f|
  = f.label :name
  = f.text_field :name
  = f.submits

/ Compiles to this if current_object.new_record? #=> true
<form action="/models" class="new_model" id="new_model" method="post">
  <div style="margin:0;padding:0">
    <input name="authenticity_token" type="hidden" value="blahblahblah" />
  </div>
  <label for="model_name">Name</label>
  <input id="model_name" name="model[name]" size="30" type="text" />
  <input id="model_submit" name="commit" type="submit" value="Create" />
</form>

/ Compiles to this if current_object.new_record? #=> false
<form action="/models/1" class="edit_model" id="edit_model" method="post">
  <div style="margin:0;padding:0">
    <input name="_method" type="hidden" value="put" />
    <input name="authenticity_token" type="hidden" value="blahblahblah" />
  </div>
  <label for="model_name">Name</label>
  <input id="model_name" name="model[name]" size="30" type="text" value="Example" />
  <input id="model_submit" name="commit" type="submit" value="Update" />
</form>