I just ran a "yum update" on my CentOS 5.3 system. After a reboot, I noticed /etc/redhat-release show CentOS 5.4.

I thought yum upgrade was supposed to do that?

I wanted to simply patch my CentOS 5.3 system without turning it into CentOS 5.4 ... YET.

Centos only supports the latest dot release of any one version. As soon as 5.4 was released there was no more support for 5.3. You can change the default behavior by changing your repo file. The default is to 5 (not 5.0 or 5.Anything) you can change that to 5.3 (or whatever) but that will lock you to that version and there will be no updates of any kind for it. This behavoir only effects the dot release jumps (5.x to 5.x+1) and not the major releases(Y.X to Y+1.X, like 4.6 to 5.0). On Centos' end the 5 is just a pointer to the most current dot release(5 used to point to the 5.3 sub directory and now it points to the 5.4 sub directory).