The basic idea is to look at the source for error_messages_for and then drop your own replacement into app/helpers/application_helper.rb.
Here's a *real* simple example:
def error_messages_for(object_name) object = instance_variable_get("@#{object_name}") return '' if object.errors.empty? content_tag('ul', object.errors.full_messages.collect {|msg| content_tag('li', msg)}, { :class => 'error' }) end