I try to keep my system up to date using the Fedora "devel" packages, using the yum updater. Sometimes I would get errors like

unsigned package //var/cache/yum/development/packages/glibc-2.3.4-3.i686.rpm
unsigned package //var/cache/yum/development/packages/eog-2.8.1-1.i386.rpm
unsigned package //var/cache/yum/development/packages/pango-1.8.0-1.i386.rpm

The problem here is that not all development packages are signed, for whatever reason. Assuming you still trust Fedora, simply modify your yum configuration file (/etc/yum.repos.d/fedora-devel.repo in my case) to add a line like

gpgcheck=0