My 2wire DSL modem has a DNS server in it that I can't shut off, and when we start heavily using the network, DNS lookups get really slow or fail completely. Lame! Sometimes I've resorted to editing my /etc/resolv.conf by hand and then doing chattr +i to keep it from changing, but that's pretty inelegant, and sucks when I want to use wireless at work.
Turns out there's a solution in the gnome network manager gui. Right click on the icon, then edit connections... wireless... edit the appropriate network... IPv4 settings... then change to "Automatic (DHCP) addresses only". Then you can put 184.108.40.206, google's public DNS server, in the "DNS servers" field.
Now, when I tried that just now, it worked fine except that the "apply" button was greyed out. I tried a bunch of things, like killing nm-applet and restarting it as root, but what finally seemed to do the trick was right clicking on the icon and unchecking "Enable networking" before doing my edits.