The interface configuration file /etc/sysconfig/network-scripts/ifcfg-<name>, where <name> refers to the name of the device that the configuration file controls.
Chapter 13.2 of the CentOS Deployment Guide said:
DNS{1,2}=<address>
where <address> is a name server address to be placed in /etc/resolv.conf if the PEERDNS directive is set to yes.
Thank you!
This put me on the right track. Turns out for Plesk at Rackspace, there are 2 NICs, and they overwrite resolv.conf sequentially. The above with PEERDNS=no set things right.