[CentOS] yum update after fresh 5.6 install fails

>    Package libuser-0.54.7-2.1.el5_5.2.x86_64.rpm is not signed

You could use --nogpgcheck but this is really weird that some packages
are not signed.
It may mean that the package is not from the trusted source, so you
should not use --nogpgcheck on a "serious" environment.