Trying to do a yum upgrade out of the box doesn't work as you don't have the required GPG keys installed. You'll get the following error:
You have enabled checking of packages via GPG keys. This is a good thing.
However, you do not have any GPG public keys installed. You need to download
the keys for packages you wish to install and install them.
Easily solved with the following command:
rpm --import /usr/share/rhn/RPM-GPG-KEY*