In newest Fedora and Centos configuration of dns is in /etc/sysconfig/network-scripts/ifcg-eth0 (or other interface config). So what for is still /etc/resolv.conf needed. Since its no longer used. (I tested this on new fedora 18 instalation and putting dns ipis in resolve.conf didn't work only changing the ifcg file and restart network service)
/etc/resolv.conf is the system-wide configuration of where to look for DNS, as used by gethostbyname() etc.
It is generally overwritten by the network service with the adapter-specific settings given in ifcfg (or provided by DHCP).
So any changes to preferred DNS servers should be made in the ifcfg files rather than resolv.conf to avoid over-writing.