rpmforge has just released a new perl-DBD-mysql for el4 that has an obsoletes against perl-DBD-MySQL and the protectbase yum plugin doesn't grok obsoletes. the priorities plugin does. so if you are having issues with this, install the yum-plugin-priorities first, make sure that the CentOS-Base repo is priority 1 and the rpmforge repo is priority 2 or higher (99 is the default). also, i'd like to suggest that the priorities plugin be made added to the base install and that the centos-base repos be configured with priority 1. it looks like c4 has the priority setting, but c5 doesn't and neither have the plugin installed. it seems like this would create a little more stable setup for people when they start adding other repos.