I install virtualbox binary 1.5.6 under gentoo 2007.0 stable.
tsarev ~ # uname -a
Linux tsarev 2.6.23-gentoo-r8 #1 SMP Thu Feb 21 07:35:32 MSK 2008 i686 Intel(R) Pentium(R) 4 CPU 3.20GHz GenuineIntel? GNU/Linux
tsarev ~ # brctl --version
bridge-utils, 1.2
tsarev ~ # cat /etc/conf.d/net
bridge_br0="eth0"
config_eth0=( "null" )
config_br0=( "dhcp" )
tsarev ~ # ifconfig
br0 Link encap:Ethernet HWaddr 00:18:F3:80:74:63
inet addr:192.168.19.210 Bcast:192.168.19.255 Mask:255.255.255.0
inet6 addr: fe80::218:f3ff:fe80:7463/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1775179 errors:0 dropped:0 overruns:0 frame:0
TX packets:2192436 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:533221882 (508.5 Mb) TX bytes:1811075264 (1727.1 Mb)
eth0 Link encap:Ethernet HWaddr 00:18:F3:80:74:63
inet6 addr: fe80::218:f3ff:fe80:7463/64 Scope:Link
UP BROADCAST RUNNING PROMISC MULTICAST MTU:1500 Metric:1
RX packets:847739 errors:0 dropped:0 overruns:0 frame:0
TX packets:1011709 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:510595453 (486.9 Mb) TX bytes:938385717 (894.9 Mb)
Interrupt:18
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:112 errors:0 dropped:0 overruns:0 frame:0
TX packets:112 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:6720 (6.5 Kb) TX bytes:6720 (6.5 Kb)
vbox0 Link encap:Ethernet HWaddr 00:FF:D3:AE:A4:40
inet6 addr: fe80::2ff:d3ff:feae:a440/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1670371 errors:0 dropped:0 overruns:0 frame:0
TX packets:1954698 errors:0 dropped:4130 overruns:0 carrier:0
collisions:0 txqueuelen:500
RX bytes:721835127 (688.3 Mb) TX bytes:1547636525 (1475.9 Mb)
This configuration (permanent bridge) and Tun/Tap birdge work good on VirtualBox 1.5.2, VirtualBox 1.5.4, but on VirtualBox 1.5.6 guest os windows xp can't discovery network and send error "limited or no connectivity connection".