It appears that Fedora core yum updating has been disabled. Perhaps this was done to try and minimize the impact on the servers, while users download the new release of Fedora Core 6?
Symptoms
# yum -y update
Loading "installonlyn" plugin
Setting up Update Process
Setting up repositories
core
[1/3]
Cannot find a valid baseurl for repo: core
Error: Cannot find a valid baseurl for repo: core
Fix
Uncomment the baseurl setting. The following was done on my FC5 systems. It makes backups, using the extension .backup.
cd /etc/yum.repos.d/ perl -pi.backup -e "s/#baseurl/baseurl/" *
Now you should be good to go.