** This tips only work for Virtual Box 1.5.4

1. Install needed software :
$apt-get install bridge-utils uml-utilities

2. Edit file /etc/network/if-pre-up.d/uml-utilities :
- replace chown root:uml-net /dev/net/tun
with chown root:vboxusers /dev/net/tun

3. Stopped networking :
$/etc/init.d/networking stop

4. Edit file /etc/network/interfaces :
(adjust accordingly )

auto lo
iface lo inet loopback

auto tap0
iface tap0 inet manual
tunctl_user user --> user login name
uml_proxy_arp 10.32.15.220 --> change according to your own ip address
uml_proxy_ether eth0 --> if your network card is eth1 then change accordingly

auto br0
iface br0 inet static
address 10.32.15.220
netmask 255.0.0.0
gateway 10.32.15.3
bridge_ports eth0 tap0
bridge_maxwait 0

auto eth0
iface eth0 inet manual

5. Start networking :
$/etc/init.d/networking start

*** Updated 29/10/2008
I've tried this trick above with VirtualBox 2.0.4 and it work like a charm :D

You only have to add this line below to work :

VBoxAddIF vbox0 [username] br0

Well let me know if it's working for you.