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
# Instead of having eth1 in /etc/network/interfaces
# we create directly br100 and attach it to eth1
auto br100
iface br100 inet static
address 172.16.0.1
netmask 255.255.0.0
network 172.16.0.0
broadcast 172.16.255.255
bridge_ports eth1
bridge_stp off
bridge_maxwait 0
bridge_fd 0
And in my devstack localrc / local.conf file I've included the following settings:
HOST_IP=172.16.0.1
FLAT_INTERFACE=br100
PUBLIC_INTERFACE=eth1
FLOATING_RANGE=172.16.0.128/25
This will allow to access your instances (when associated to a floating IP) from your host machine
Hope this works for you too!
Everett