A Ruby implementation of the CAS client is now available at http://rubyforge.org/projects/rubycas-client/
The library is designed to easily integrate with Rails as an ActionController filter, but may be adapted for other purposes.
Documentation and example usage is available at http://rubycas-client.rubyforge.org/
The easiest way to install the client is via ruby gems, by typing the following at a shell prompt (you will probably need root access):
gem install rubycas-client