console

>> a = Post.first(:conditions => { :published => true })
=> #<Post ... >
>> a.update_attributes(:title => "test 1234")
=> false
>> a.errors
=> #<ActiveRecord::Errors:: ... @errors => {"title" => ["translation missing: ... frozen_attribute_when_published"

>> a = Post.first(:conditions => { :published => false })
=> #<Post ... >
>> a.update_attributes(:title => "test 1234")
=> true

custom_validations.rb

module ActiveRecord
  module CustomValidations
    def validates_unchanged_value(*attr_names)
      configuration = { :message => :frozen_attribute }
      configuration.update(attr_names.extract_options!)
      configuration[:on] = :update

      validates_each(attr_names, configuration) do |record, attr_name, value|
        record.errors.add(attr_name, :frozen_attribute, :default => configuration[:message]) if record.send("#{attr_name}_changed?")
      end
    end
  end
end

ActiveRecord::Base.send(:extend, ActiveRecord::CustomValidations)

post.rb

class Post < ActiveRecord::Base
  #...

  validates_unchanged_value :title, :if       => published?,
                                    :message  => :frozen_attribute_when_published
end