UPDATE!  RHEL/CentOS 5.5 includes FreeRadius2,  you no longer need to include the external repository to get the freeradius2 packages.   I’ve updated the post to reflect this fact.  CentOS 5.4 users will have to fetch the freeradius2 source rpms and build the packages themselves. 🙁
Up to version 5.4, RHEL/CentOS  shiped with FreeRadius 1.1.x, a version long in the tooth.  Anyone trying to do anything with FreeRadius 1.x will quickly discover that they are out of luck when it comes to documentation and support.
As of RHEL/CentOS 5.5, Freeradius 2.x is available via the freeradius2 packages.  The FreeRadius  1.x packages are still available if you need them  You cannot install version 1 and version 2 on the same host at the samet ime!
Here are the steps to install FreeRadius2 on CentOS5.

  1. To see the list of available packages, run
    # yum search freeradius2
    Loaded plugins: fastestmirror
    Loading mirror speeds from cached hostfile
    * addons: mirror.csclub.uwaterloo.ca
    * base: mirror.skiplink.com
    * extras: mirror.csclub.uwaterloo.ca
    * updates: centos.omnispring.com
    freeradius2 | 1.2 kB 00:00
    freeradius2/primary | 43 kB 00:00
    freeradius2 149/149
    ============================================ Matched: freeradius2 ============================================
    freeradius2.i386 : High-performance and highly configurable free RADIUS server
    freeradius2-debuginfo.i386 : Debug information for package freeradius2
    freeradius2-devel.i386 : FreeRADIUS Development Files
    freeradius2-krb5.i386 : Kerberos 5 support for freeradius
    freeradius2-ldap.i386 : LDAP support for freeradius
    freeradius2-mysql.i386 : MySQL support for freeradius
    freeradius2-perl.i386 : Perl support for freeradius
    freeradius2-postgresql.i386 : Postgresql support for freeradius
    freeradius2-python.i386 : Python support for freeradius
    freeradius2-unixODBC.i386 : Unix ODBC support for freeradius
    freeradius2-utils.i386 : FreeRADIUS utilities
  2. Install the the packages you want. You’ll at least want to install freeradius2, freeradius2-utils.# yum install freeradius2 freeradius2-utils freeradius2-ldap
    Loaded plugins: fastestmirror
    Loading mirror speeds from cached hostfile
    * addons: mirror.csclub.uwaterloo.ca
    * base: mirror.skiplink.com
    * extras: mirror.csclub.uwaterloo.ca
    * updates: centos.omnispring.com
    freeradius2 | 1.2 kB 00:00
    Setting up Install Process
    Resolving Dependencies
    --> Running transaction check
    ---> Package freeradius2.i386 0:2.1.8-2.el5 set to be updated
    ---> Package freeradius2-ldap.i386 0:2.1.8-2.el5 set to be updated
    ---> Package freeradius2-utils.i386 0:2.1.8-2.el5 set to be updated
    --> Finished Dependency Resolution

    Dependencies Resolved
    ===========================================================================================================================================
    Package Arch Version Repository Size
    ===========================================================================================================================================
    Installing:
    freeradius2 i386 2.1.8-2.el5 freeradius2 1.5 M
    freeradius2-ldap i386 2.1.8-2.el5 freeradius2 49 k
    freeradius2-utils i386 2.1.8-2.el5 freeradius2 118 k
    Transaction Summary
    ===========================================================================================================================================
    Install 3 Package(s)
    Update 0 Package(s)
    Remove 0 Package(s)
    Total size: 1.6 M
    Is this ok [y/N]: y
    Downloading Packages:
    Running rpm_check_debug
    Running Transaction Test
    Finished Transaction Test
    Transaction Test Succeeded
    Running Transaction
    Installing : freeradius2 1/3
    Installing : freeradius2-utils 2/3
    Installing : freeradius2-ldap 3/3
    Installed:
    freeradius2.i386 0:2.1.8-2.el5 freeradius2-ldap.i386 0:2.1.8-2.el5 freeradius2-utils.i386 0:2.1.8-2.el5
    Complete!

That’s it. If you have never setup radius before I recommend you start with Setting up FreeRADIUS for the first time.
The primary source of documentation is the FreeRadius documentation page. A second valuable source of documentation site is DeployingRadius.com.
Enterprise Networking Planet has a series of articles on using FreeRADIUS for Wi-Fi Authentication. If you decide to follow the articles, just remember that you no longer need to install the third party repo.