Just last month, this fashionable San Diego enclave was ground zero in the war against urban sprawl. In one of the United States' most destructive acts of environmental terrorism, investigators say, members of a group calling itself the Earth Liberation Front crept onto a construction site here in the middle of the night and set it ablaze. They left a banner that read: "If you build it, we will burn it. The E.L.F.s are mad."