form_builder_extensions.rb

# Chuck this in config/initializers for view-first validations
ActionView::Base.default_form_builder.class_eval do
  def error(field)
    case err = object.errors.on(field)
    when String: err
    when Array: err.first
    end
  end
  def errors(field)
    case err = object.errors.on(field)
    when String: [err]
    when Array: err
    when Nil: []
    end
  end
end

register.haml

-# And you can use it in a view like so:

- form_for :user, @user, register_path do |f|
  %fieldset
    != f.label :name
    != f.text_field :name
    - if f.error(:name) == "blank"
      .error.name_blank You don't have a name??
      
    != f.label :email
    != f.text_field :email
    - case f.error(:email)
    - when "blank"
      .error.email_blank You don't have an email??
    - when "format"
      .error.email_format That doesn't look like an email address!
    - when "taken"
      .error.email_taken
        There is an account already registered with that email address.
        %a{:href => root_path(:email => @user.email)} Log in instead?

user.rb

class User < ActiveRecord::Base
  validates_presence_of :email, :name, :password, :message => "blank"
  validates_format_of :email, :with => /^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$/i, :allow_blank => true, :message => "format"
  validates_uniqueness_of :email, :case_sensitive => false, :message => "taken"
end