Most people will find that installing CAS 3.0.x on RHEL 5 isn't initially as straightforward as they hoped. This is largely in part due to Redhat not officially supporting either the Sun Java Development Kit (JDK) or Java Runtime Environment (JRE). By default, Redhat has bundled RHEL 5 with the GNU Compiler for Java (GCJ) from JPackage, which is equivalent to J2SE 1.4.2 as of July 2007. This can cause code compiled by newer versions of Sun JDK / JRE to be incompatible with GCJ. However, it is not only possible but fairly easy to get CAS 3.0.x deployed on RHEL 5.
NOTE: This walkthrough involves the use of precompiled binaries rather than keeping RHEL 5 in sync with yum repositories.