Temporary solution:

1. sudo apt-get install uml-utilities bridge-utils
2. sudo tunctl -t tap1 -u USER_OF_VBOX_ON_HOST
3. sudo brctl addbr br0
4. sudo ifconfig eth0 0.0.0.0 promisc
5. sudo brctl addif br0 eth0
6. sudo dhclient br0
   OR
   sudo ifconfig br0 STATIC.IP.OF.HOST
7. sudo brctl addif br0 tap1
8. sudo ifconfig tap1 up
9. sudo chmod 0666 /dev/net/tun
10. At the VirtualBox startup panel, choose “Host Interface” and add “tap1″ to “Interface Name”.
11. At the guest (after boot up the guest OS), change the IP of the guest OS to the same subnet of your host:
    IP 192.168.1.200
    netmask 255.255.255.0
    gateway 192.168.1.1
    DNS #1 208.67.222.222
    DNS #2 208.67.220.220
12. sudo route add default gw IP.OF.DEFAULT.GATEWAY

Source and details on: http://samiux.wordpress.com/2007/07/11/bridge-network-interface-on-virtu...

Permanent solution:

sudo apt-get install uml-utilities bridge-utils
sudo invoke-rc.d networking stop
sudo nano /etc/network/interfaces

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet manual

auto br0
iface br0 inet dhcp
bridge_ports eth0
bridge_fd 9
bridge_hello 2
bridge_maxage 12
bridge_stp off

sudo /etc/init.d/networking restart

At the VirtualBox startup panel, choose “Host Interface” and add “br0″ to “Interface Name”.