custom errors for validates_presence, etc..

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