I ran into the following warning while trying to install a package from the IUS third-party repository for CentOS:
warning: ius-release-1.0-6.ius.el5.noarch.rpm: Header V3 DSA signature: NOKEY, key ID 9cd4953f.
This was caused because the IUS GPG key was not (correctly) installed. To fix, type:
sudo yum install yum-plugin-replace
This should prompt you to download and install the correct key. Once this is done, the installation of other packages should proceed smoothly.
Hope this helps :)