I've been toying around with Rails3 the past few weeks. My current Gemfile looks like this:

 gem 'rails', '3.0.0.beta4'
 gem 'capybara'
 gem 'database_cleaner'
 gem 'cucumber-rails'
 gem 'cucumber'
 gem 'rspec-rails', '2.0.0.beta.11'
 gem 'rspec', '2.0.0.beta.11'
 gem 'haml'
 gem 'spork'
 gem 'launchy'
 gem 'mongoid', "2.0.0.beta6"
 gem 'bson_ext'
 gem 'ruby-debug'
 gem 'machinist_mongo', :require => 'machinist/mongoid'
 gem 'faker'