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. 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
It can also be installed into a Rails application as a plugin:
script/plugin install http://rubycas-client.googlecode.com/svn/trunk/rubycas-client
Documentation and example usage is available at http://rubycas-client.rubyforge.org/
CAS Plugin
A second implementation is available as a plugin.