R is in EPEL (Extra Packages for Enterprise Linux):
EPEL (Extra Packages for Enterprise Linux) is a volunteer-based community effort from the Fedora project to create a repository of high-quality add-on packages that complement the Fedora-based Red Hat Enterprise Linux (RHEL) and its compatible spinoffs, such as CentOS and Scientific Linux.
As part of the Fedora packaging community, EPEL packages are 100% free/libre open source software (FLOSS).
All you need to install R is to add the EPEL repository and then install R the same way you do with any other package:
rpm -Uvh http://download.fedoraproject.org/pub/epel/5/i386/epel-release-5-4.noarch.rpm
yum install R