Scott Harvey asked on the Rails Australia list how to do client and server-side validation in one hit. Seeing as Rails has no public documented API for validation reflection, here’s an example of how you could roll your own required fields designation DSL:

require 'rubygems'
require 'active_record'

module ActiveRecord
  # DIY required field reflection, with auto +validates_presence_of+.
  # 
  # Adds a class method +required_fields+ that returns the array of required fields.
  module RequiredFields
    def self.included(mod)
      mod.extend ClassMethods
      mod.class_inheritable_array(:required_fields)
      mod.write_inheritable_array(:required_fields, [])
    end
    module ClassMethods
      # Adds the specified +fields+ to +required_fields+ and sets up the corresponding +validates_presence_of+ validations
      def requires_fields(*fields)
        validates_presence_of(fields)
        write_inheritable_array(:required_fields, fields)
      end
    end
  end
end

ActiveRecord::Base.send(:include, ActiveRecord::RequiredFields)

if $0 == __FILE__
  require 'test/unit'
  
  class RequiredFieldsTest < Test::Unit::TestCase
    class NoRequiredFields < ActiveRecord::Base
    end
    class OneRequiredFieldOneStatement < ActiveRecord::Base
      requires_fields :body
    end
    class MultipleRequiredOneStatement < ActiveRecord::Base
      requires_fields :body, :description
    end
    class MultipleRequiredMultipleStatements < ActiveRecord::Base
      requires_fields :body, :description
      requires_fields :created_at
    end    
    def test_required_fields
      assert_equal [], NoRequiredFields.required_fields
      assert_equal [:body], OneRequiredFieldOneStatement.required_fields
      assert_equal [:body, :description], MultipleRequiredOneStatement.required_fields
      assert_equal [:body, :description, :created_at], MultipleRequiredMultipleStatements.required_fields
    end
    # I'd test the actual field validation too, but that needs a (mocked, or otherwise) DB connection
  end
end

In your view you then just need to check if the field is required and mark it as so.