How to add a line like:
/etc/resolv.conf on Network Manager startup in Ubuntu?
NetworkManager uses dhclient for DHCP leases, and you would be better off using dhclient hooks to change /etc/resolv.conf
You're probably running dhcp3-client on Ubuntu, I think it's the default with 8.10 and 9.04 at least.
If you are, your configuration file is in /etc/dhcp3/dhclient.conf
You can specify, per-interface modifications to /etc/resolv.conf
That will generate an /etc/resolv.conf with:
man 5 dhclient.conf for more info.