Clem,
Your code needs to look like this:
<%= form_remote_tag :url => { :action => :add_to_cart, :id => product } do %> <%= submit_tag "Add to Cart" %> <%= end_form_tag %>
The <% ... %> tags will evaluate the code between them without outputting the result into the HTML. The <%= ... %> tags, on the other hand, will output the result.
Hope that helps, Matt Buck