I spoke with z00dax on #centos-devel about a couple things.
1. For a list of packages/modified add from RHEL base on CentOS you can see
http://wiki.centos.org/Manuals/ReleaseNotes/CentOS5.0
I read through it and only saw a few yum things jump out at me and I
*think* they have all been addressed.
2. The CentOS folks seem to want to work together when packages
require updates. This mostly is from CentOS Plus , Extras but also
could apply to the base OS. So, if you're an EPEL maintainer and
CentOS has a package, it wouldn't hurt to ping them with what/why
you're updating.
<z00dax> if there is a package that comes via centos, and someone
thinks it needs an update / change / fix - we all gain by fixing it
everywhere
stahnma