I’m working to integrate an application on a remote-hosted IIS server into our CAS environment. CASisapi (svn trunk or svn tags/production) may do the trick, though Phil Sladen struggled with it (in 2005). There’s reason to doubt it. Not only is the sparse information all old, I first learned about it from a page full of broken links and the apparent author recommends against it. There’s a little more information here for those who can read Danish.
UC Davis’ CAS ISAPI client may be a better solution (it certainly looks easy to install). Builder AU talks about .NET + CAS, and Case Western has a lot of documentation. Only partially related: it looks like World of Warcraft uses CAS.