[CentOS] yum update after fresh 5.6 install fails

Hi,
I just finished installing a CentOS 5.6 x86_64 system and the first
thing I wanted to do after the install is to apply the software patches.

But I get the following error when I try to run 'yum update':

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

and yum update exits without applying any of the patches.


I tried clearing the cache and re-running the update command but I get
the same error.

Any help with this would be much appreciated.

Best regards,
  Andre