Local and DHCP provided DNS servers behave differently with Snow Leopard.  With previous versions of Mac OS X, if you had one or more DNS servers entered into the Network system preference pane, and you were also getting DNS server info from a DHCP server, any DNS entries in the Network system preference pane would be used in ADDITION to those specified by DHCP.  With Snow Leopard, if you have one or more DNS servers entered in the Network system preferences pane, they are used exclusively, and any DHCP specified DNS servers are ignored.