Is it possible that your default route got changed or eliminated? We had a system where we installed fc12 and then immediately an "yum update" (was building for lab testing) and the next day went to install a another tool using yum and got this error. Tried a number of things. After investigating found that the default route was gone and so the box could not get off the local network.
Interestingly enough it was able to identify that the packages were available but it could not then subsequently fetch them from any mirror.
You can check your default route like this (from the command line):
[ddb@dbsprod ~]$ netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
The "Gateway" should not be
, it should be your default router, in my case
Once I fixed that it worked fine. You can make the changes through NETWORK PREFERENCES as root through the GUI or through the command line.
Once we fixed this it worked fine. Not sure why it got changed to begin with.