I started with the guide Experimenting with OpenStack Essex on Ubuntu 12.04 LTS under VirtualBox and here are the settings I wound up with in VirtualBox

Under Preferences > Network create a Host-only network called vboxnet0.

vboxnet0 (disable DHCP Server too)

Create a new VM and go to the Network section.

The Adapters for the VM itself.

Network settings in the Ubuntu VM /etc/network/interfaces

auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet dhcp

#Public Interface
auto eth1
iface eth1 inet static
address 172.16.0.1
netmask 255.255.0.0
network 172.16.0.0
broadcast 172.16.255.255

And in my devstack localrc file I've included the two settings.

FLAT_INTERFACE=br100
PUBLIC_INTERFACE=eth1

Hope this works for you too!

You can also use this Vagrant recipe - which will do the same steps

https://github.com/bcwaldon/vagrant_devstack