I was doing some network reconfiguration on a box recently. I was switching from a static IP setup (which was originally configured during install) to a DHCP setup.
Problem was, regardless of what I did, I could not get the default route from the DHCP server to apply. I end up with no default route after getting an address.
A lot of googling, mailing list asking, and IRC later, I finally discovered that the file /etc/sysconfig/network had this line in it:
GATEWAY=<some ip address>
It must have been put there during the original install.
Removing that and restarting networking caused the machine to properly apply the default route give to it by the DHCP server.