What Redhat is now also going to offer in about 6 months time, is the ability for people to just stay within one branch of the release ( so, people who installed 5.1 might want to stay within the 5.1 package set and not get any enhancements, or bugfix's or package updates - they just get a very small subset of security fixs ). And to do that you would need to put your machine into a Branch, which will be called 5.x.<something>. ( eg. for the entire life of 5.1.* your redhat-release file on centos is going to say : CentOS release 5.1.z (Branch) )
If you still dont get it, dont worry - just sit back for 6 months. The situation will hopefully clear itself out in that time. There is nothing that needs to be done now, and there is no 5.0.* Branch.
I don't see a great benefit for more than a very small subset of users with this. Most people shouldn't even worry about it. If you need this option, you probably are already paying a support contract, and your feedback probably got the ball rolling in the first place.