For a few months now, I've been having an odd issue on OS X where my /etc/hosts file keeps getting reset to some default. It was a nuisance and I basically resorted to some scripts to update my hosts file and restore the correct settings.
Finally, spent some time investigating this and the mystery is now solved
Turns out that if you have installed the Cisco AnyConnect VPN, it creates an /etc/hosts.ac file and, on reboot, restores that file as the /etc/hosts file.
Simple fix is to modify the /etc/hosts.ac file instead. So basically, I've traded one kludge fix for another one.