After a FC4 glibc update a couple weeks ago, I've found that yum updates stopped working. When I went to debug it, I eventually found that the "lesser" programs like ping and ftp worked, while the "higher-level" programs didn't; like curl, wget, ncftp, and yum.
I tracked this problem back to libresolv (the former programs didn't use it, while the latter did). Eventually I found thread on FedoraForum.org about dns problems after a glibc update as well. Turns out the updated nsswitch.conf didn't work with some old config. Grr.