I spent some time trying to get rails 3 beta 2 running on Ubuntu.
The default gem install using apt-get is 1.3.5.
For rails 3 you will need gem 1.3.6.
Install RubyGems 1.3.6
OK now you are ready to follow the rails guide:
Ok Now to install rspec:
If you don't have xmllib already setup you will get errors building nokogiri. Follow these to fix that:
Then install rspec-rails 2.0:
Happy Rails 3 Riding!