nice_field_errors.rb

# Place in /config/initializers
ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
  if html_tag =~ /<(input|textarea|select|label)[^>]+style=/
    style_attribute = html_tag =~ /class=['"]/
    html_tag.insert(style_attribute + 7, "fieldWithErrors ")
  elsif html_tag =~ /<(input|textarea|select|label)/
    first_whitespace = html_tag =~ /\s/
    html_tag[first_whitespace] = ' class="fieldWithErrors" '
  end
  html_tag
end