Update 16 Oct 2013: VirtualBox 4.3 provides a fix. I’ve tried it out, and it does work although we get some warnings about unknown kernel extensions. See this comment: https://www.virtualbox.org/ticket/8940#comment:10
I’ve left the trail of progress here in case you can’t upgrade, or just don’t want to …
Update 8 Oct 2013: Upgrading to OS X Mavericks GM, ran into some more problems, so I’v been issuing:sudo /Library/StartupItems/VirtualBox/VirtualBox restartto fix it, see also this VirtualBox ticket : https://www.virtualbox.org/ticket/8940.
Original Post 2 Oct 2013: Just a quick note on OS X Mavericks, I’m using Developer Preview 8, and the fix provided by theengguy worked for me: http://theengguy.blogspot.com/2013/08/virtualbox-and-bridged-network-on-os-x.html
The symptom is that, upon startup, bridged network guest (in my case, both Ubuntu 12.04 and 13.04) throw an error (which I forgot exactly what now, but something to do with the network adapter). It’ll run just fine in NAT mode.
Any way, thanks thengguy, solution worked perfectly!