NSA guides on hardening RHEL5. Should be applicable to CentOS5 as well. http://www.nsa.gov/snac/downloads_redhat.cfm?MenuID=scg10.3.1.1
I read about this on /. some weeks ago, but I just skimmed through it, so I can't say how effective I think it is. I thought it would be useful to point to it on list though.
HTH, Filipe