8 month after RHEL 6 was released the free clone CentOS 6 is released. It includes all new features and is binary compatible to Red Hats Enterprise Linux 6.
Based on Linux kernel 2.6.32 it includes for example PostgreSQL 8.4 and MySQL 5.1.
Like RedHat, the CentOS team will release 7 Years patches and service packs for this distribution release.
It's not recommended to update your CentOS 4 or 5 installation. You should do a new installation if you plan to migrate your software.
Here you can find the complete release notes for Centos 6.0.