Initial install done using great How To on Contribs.org. After the initial installation if you want to upgrade VirtualBox or the SME kernel is upgraded you need to rebuild VirtualBox against the SME kernel.
Get the VirtualBox rpm you need (for SME 8 beta 7 it is RHEL5/CentoOS 5 version) from VirtualBox.org.
Gently reinstall (as opposed to Force)
rpm -vi --replacepkgs VirtualBox-4.1-4.1.8_75467_rhel5-1.i386.rpm
That should take some time and rebuild what you need.
Edit the following file: /etc/vbox/vbox.cfg so it contains:
VBOXWEB_USER='root' VBOXWEB_HOST=127.0.0.1 VBOXWEB_PORT=18083
If you upgraded VirtualBox then get the latest extensions too:
wget http://download.virtualbox.org/virtualbox/4.1.2/Oracle_VM_VirtualBox_Extension_Pack-4.1.2-73507.vbox-extpack vboxmanage extpack install Oracle_VM_VirtualBox_Extension_Pack-4.1.2-73507.vbox-extpack
Finally for safe measures:
signal-event post-upgrade signal-event reboot