OSX 10.9 Mavericks VirtualBox unable to use Bridged Networking
I've found a temporary solution.  It appears that on reboot not all the kernel extensions are loaded correctly.  However a restart of the VirtualBox startup item seems to resolve this and I can then start the machines.

sudo  /Library/StartupItems/VirtualBox/VirtualBox restart


Is the line that works for me, sometimes I have to run it twice.