In my app I simply have a submit_to_remote within the larger form... no need for a nested form. It submits the entire form including the other fields but I only read the category[:name] param when I receive it in my CategoriesController#create action.
Inside a larger form... <%= text_field_tag 'category[name]' %> <%= submit_to_remote 'add_category', 'Add', :url => categories_path %>
I belive there is a :submit parameter available for submit_to_remote that should specify which field(s) to submit instead of the whole form, but I haven't played with that. (See the API doc for link_to_remote: as I understand it, submit_to_remote uses the same options.)