I’ve been pulling my (few) hairs out on this one, but when DHCP fails to work on some networks, it could mean you didn’t allow configd to accept incoming connections. When you connect to a new network, at start up, MacOS (at least in MacOS 10.5/Leopard) will ask you to allow or disallow incoming connections to configd. Allow it, otherwise it will not work!
Well, that’s what I think.. The only solution I have right now is to disable the firewall.. If anyone can tell me how to add configd to the firwall, would be great!