again not trying to get in a pissing match, but CentOS 5.x and 6.x are not 100% RHE.
one example for added features to CentOS that are directly from Fedora, not RH, is the EPEL that is required for many features not provided by CentOS directly. While CentOS does not produce, nor support EPEL, here it is directly from their wiki and pointing to Fedoraproject.org.
Extra Packages for Enterprise Linux (EPEL) - This repository (See http://fedoraproject.org/wiki/EPEL) provides rebuilds of Fedora packages for EL5 and EL6. Install the appropriate epel-release package for EL5 or EL6 to configure.
This repo has made efforts not to replace system packages. In some cases it has endeavored to directly address CentOS compatibility but has expressly denied inter-repository compatibility as a goal. It may not mix well with other 3rd party repos. So, make SURE you are using the Priorities yum plugin if you are using EPEL ... especially if you mix its packages with those from other 3rd party repos. It should also be noted that, while EPEL may not overwrite distro packages, it may have conflicts with the CentOS extras repo which is enabled by default.
Mailing list for EPEL development - not really a support list, but seems to be the only option. On that list, an EPEL community member has indicated that attending to CentOS support and compatibility (to 'cater for' CentOS) was not an EPEL responsibility to him, as the CentOS usage level was not "verified with a realistic statistic". Another at EPEL recalled the history and represented a 'cooler head' with a goal to not gratuitously break CentOS with version skew.
also note my quote for termination of end of life cycle is current from CentOS FAQ, they will not be changing those dates any time soon for the exact reason I pointed out earlier. The extended life cycle provided by RH is a paid for service. The end of life cycle for all intensive purposes is currently 5.5 years. Thus CentOS 5.x will terminate in 2017 with CentOS 6.x lasting until 2020, thus a much better option for fresh install over the 5.x line.
again not trying to create a pissing fight, just pointing out some basic facts.
1. CentOS 5.x and 6.x are not 100% bitwise compatible with RHE. CentOS 2.x, 3.x, and 4.x were.
2. The current life cycle of CentOS is roughly 10 years that means CentOS 5.x will come to the end of its life cycle soon, not as soon as I had thought as I had its start date in my head os much earlier then it really was. As CentOS 5.x went live shortly after RHEL 5.x in 2007, that means CentOS 5.x will end its life support cycle in 2017, roughly 3 years from now. I had it in my head that CentOS 5.x started in 2003 or 2004, thus its end of life cycle would be much sooner then it really is.
3. CentOS is IMHO a better option then Ubuntu for a server OS due to its life cycle being much longer.
Are any of those points wrong? No they are not.
I have provided a link from CentOS directly on they are no longer 100% bit compatible with RHE since v 4.x
The support life cycle of CentOS 5.x will terminate in 2017 according to the link provided from CentOS FAQ page.
The life cycle of CentOS is far greater then that of Ubuntu, again in my opinion that makes it the better choice for a server OS. you may not like that, but it is an opinion and you are welcome to discus as to why you dont think that CentOS is a better choice then Ubuntu for a server class OS.