While people are still waiting to for Centos 6 (after many month since RHEL6 was released) we got a little teaser in the release of Centos 5.6 last week. This is a minor security release, but it also brings in some interesting additions like ext4 support and php53 packages among others.
If you are running any Centos 5.x minor version you should be able to upgrade just by running:
We already upgraded to Centos 5.6 all our servers and you should do the same also, and hopefully soon we will see the Centos6 release that everyone is waiting for.
For the full list of packages changed/added please see the centos5.6 release notes: http://wiki.centos.org/Manuals/ReleaseNotes/CentOS5.6