Comment 1 for bug 24826

(In reply to comment #0)
> If a users wants to add other nameservers to /etc/resolv.conf, dhcpclient3
> deletes them on the next dhcp update. Instead it should merely replace the
> $old_name_servers with the $new_name_servers in the same relative order with
> whatever is currently there. If /etc/resolv.conf is empty, it should add the
values.

Just put this line in /etc/dhcp3/dhclient.conf :
---------------------
prepend domain-name-servers XXX.XXX.XXX.XXX,YYY.YYY.YYY.YYY,ZZZ.ZZZ.ZZZ.ZZZ;
---------------------
to preserve your change...

I think it is impossible to distinct $old_name_servers and
$manually_added_name_servers.