I recently installed Ubuntu 12.10 on my new laptop and ran into an issue with VirtualBox. Apparently there is a bug with NAT. If your guest OS is not resolving DNS (or appears to not reach the internet), you can either download and install 4.2+ from Oracle or try this:
VBoxManage modifyvm "name" --natdnshostresolver1 on VBoxManage modifyvm "name" --natdnsproxy1 on
It forces the guest OS to use the host operating system’s DNS, which is handy. It worked for me. Your mileage may vary.