After I upgraded my GNU/Linux system from FC4 to FC5, yum update stopped working. This is now fixed.
yum couldn't find repositories, it couldn't read repomd.xml files, it really couldn't do anything at all. Firefox would load a repomd.xml off the net just fine, but wget would act as though the file didn't exist.
I traced the problem (in my case) to /etc/nsswitch.conf:
< hosts: files nisplus dns
> hosts: files dns