Actually, bring Arel back to Gemfile and use git repo since it helps development.

Gemfile

 path File.dirname(__FILE__)
 source 'http://rubygems.org'
 
+gem "arel", :git => "git://github.com/rails/arel.git"
 gem "rails", "3.0.0.beta1"
 
 gem "rake",  ">= 0.8.7"