FYI
If you see "yum update" failing with a message
Cannot find a valid baseurl for repo: updates-released
this is because of a server maintenance on Red Hat's side. The mirror selection page is actually affected and out of order.
http://fedora.redhat.com/download/mirrors/updates-released-fc4 ( same for FC3)
The page now shows:
Site Maintenance In order to provide you with a better experience on redhat.com we are currently doing site maintenance and upgrading our systems. We apologize for any inconvenience this may cause. Please come back in a few hours for full access to our improved site.
To contact Red Hat Support please call 1-888-RedHat1.
So don't be alarmed and think your yum is broken by doing nothing ;)
Alexander