Oh yeah, two other handy networking related commands I use are:

  vb.customize ["modifyvm", :id, "--macaddress1", "auto"]

Which forces a fresh MAC address.

  vb.customize ["modifyvm", :id, "--nictype1", "Am79C970A"]

Which changes the virtual NIC type (for whatever reason, I find on my current host Am79C970A gives much better transfer speeds than virtio)