put this in your test_helper.rb

  def test_all_models_valid model
    objs = model.camelize.constantize.find(:all)
    objs.each do |obj|
      assert obj.valid?, "#{model} #{obj.id} is invalid: #{obj.errors.full_messages.join(', ')}" 
    end
  end

  def test_all_models_valid! filename
    test_all_models_valid klass = filename.split('/').last.sub(/_test.rb/,'').camelize
  end

  def test_valid_fixtures
    test_all_models_valid! __FILE__
  end

Sweet! :)

Update #1: Thanks to Nicolas Sanguinetti for the tip on how to avoid the use of eval.