Hi All,
I apologize if this is an inappropriate place to post such things but I recall seeing messages as recently as last month from people having difficulties using Alex Li's rc-cas-plugin (http://code.google.com/p/rc-cas-plugin/) so I thought this may be of interest to them.
I have created a fork of his plugin that works with stock Roundcube 0.6 and greater. It can be found here:
https://github.com/dfwarden/Roundcube-CAS-Authn
Most of the relevant info is in the README and the default config file.
We have been using this plugin since the beginning of 2012 on our production Roundcube cluster without a problem. (8 RHEL 6.2, RC 0.7 hosts behind LVS load balancer)
Hope this helps, David Warden SUNY Geneseo