_product.html.erb

<!-- _product.html.erb -->
<% div_for(product) do %>
  <p id="<%= dom_id(product, "price") %>"><%=h product.price %></p>
  <!-- periodic update ... -->
  <%= image_tag(product.photo.url(:thumb), :id => dom_id(product, "photo")) %>
<% end %>

index.html.erb

<!-- index.html.erb -->
<%= render @products %>

products_controller.rb

def update
  @product = Product.find(params[:id])
  @product.update_attributes(params[:product])
  respond_to do |wants|
    wants.html { redirect_to products_url }
    wants.js do
      render :update do |page|
        # something like this ...
        page[dom_id(@product)].replace :partial => @product
        # OR maybe something like this ...
        page[dom_id(@product, "price")].replace_html @product.price
      end
    end
  end
end