(note to listadmin -- is the default Reply-To the sender, not the list?)
syv wrote:
rpm --import http://mirror.centos.org/centos/3.1/i386/RPM-GPG-KEY-CentOS-3
I ran this earlier today:
rpm --import http://ftp.gui.uva.es/sites/caosity.org/centos-3/3.1/i386/RPM-GPG-KEY-CentOS...
and was able to continue onwards, installing extra stuff (kernel-smp eg). Perhaps what downloaded from your mirror got a little scrambled? Try that source I used perhaps?
-te