The CentOS team just released their latest version of CentOS-5.2, a minor upgrade for the Centos-5 release. This is based on the upstream release RHEL 5.2.0 and includes packages from all variants including server and client repositories.
Any system running CentOS-5 (either 5.0 or 5.1) can be upgraded by just running:
that will bring in the new packages from 5.2 and update the system. Depending on the size of your install and your connection speed this can take a while to complete ;-) .