Here is how the error looks like when you run 'yum install subversion':

--> Missing Dependency: libpq.so.4

First what is libpq.so.4? Generally it is part of Postgre (and its related packages) which is of no further interest in our case

Second, if you don't use Postgre you can simply remove it with yum to save additional complications:

#yum remove postgresql

Now comes the most important part - remove the extra centos repositories. For this purpose make sure that there is only one depository file left in /etc/yum.repos.d/CentOS-Base.repo and there leave only the main repository:

name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5

Then run again yum install subversion and you are in the business

A word from me: the biggest problem to CentOS is its unresolved dependencies from different repositories. That's probably the only difference with RedHat.