On my Red Hat 5.5 box (similar to CentOS), I was trying to install Virtualmin.

I got two errors while installing.

-This system is not registered with RHN.
RHN support will be disabled.
\Error: Missing Dependency: ruby-rdoc is needed by package rubygems-0.9.2-1.el5.noarch (virtualmin)
Error: Missing Dependency: perl(XML::Parser) is needed by package perl-XML-Simple-2.14-4.fc6.noarch (virtualmin)
Error: Missing Dependency: libdistcache.so.1()(64bit) is needed by package 2:mod_ssl-2.2.3-43.3.vm.x86_64 (virtualmin)
Error: Missing Dependency: libaprutil-1.so.0()(64bit) is needed by package 1:httpd-2.2.3-43.3.vm.x86_64 (virtualmin)
Error: Missing Dependency: libnal.so.1()(64bit) is needed by package 2:mod_ssl-2.2.3-43.3.vm.x86_64 (virtualmin)

This simply meant that I had to run rhn_register to register using the Red Hat login/password that I had received. Because you cannot run yum/update without registering with RHN, unlike CentOS which can update without registering. Simple enough.

Error: Missing Dependency: libGeoIP.so.1 is needed by package proftpd
This happens because apparently Virtualmin updated to proftpd from 1.3.2 to 1.3.3 without testing properly (source: Virtualmin Forums). To fix, I installed RPMForge and ran install script again.