Editted solution in case people don’t read the reactions:
<%= form_for [:admin, @person, @image] do |f| %>
Old response:
I have a project with an admin namespace and People and Images resources, this is the way I build my form_for in rails3, I haven’t found a way just yet to do it cleaner…
<%= form_for [@person, @image], :url => admin_person_images_path do |f| %>