I installed a brand new copy of Fedora 16 today on our work network so I can experiment with adding a Linux box onto a Windows network.
Web browsing was fine (after I configured the proxy settings in Firefox!) But any system / software updates did not work.
After many attempts and many search terms in Google, I found the solution.
Open a terminal window and log in as the su.
<hit ESC>then type in ZZ (this will save and quit)
for more information see http://fedora.redhat.com/docs/yum/sn-yum-proxy-server.html