In Ubuntu 12.04 the DNS handling ahs changed. The data in /etc/resolv.conf is overwritten at every network change and the resolvconf scripts are used.
To find out some information about the current name servers you can use nm-tool.
If you are using a vagrant box with precise you can add this to your Vagrantfile in order to fix the network problems:
config.vm.customize ["modifyvm", :id, "--natdnsproxy1", "on", "--natdnshostresolver1", "on"]
More information can be found in this blog post.