From the section about stylesheets in the Formtastic README:
Rails 3.1 introduces an asset pipeline that allows plugins like Formtastic to serve their own Stylesheets, Javascripts, etc without having to run generators that copy them across to the host application. Formtastic makes three stylesheets available as an Engine, you just need to require them in your global stylesheets.
# app/assets/stylesheets/application.css
*= require formtastic
*= require my_formtastic_changes`
Perhaps that's what you're looking for.