VirtualBox sources currently (2011-05-15) hardcodes library RPATH to /opt/VirtualBox.
If the 9999 ebuild does not specify the installation path beforehands, there will be a mismatch, and /usr/lib/virtualbox/VirtualBox.so will try to load VBoxVMM.so from /opt/VirtualBox.
Upstream suggested solution is to perform :
echo "VBOX_PATH_APP_PRIVATE = /usr/lib/virtualbox" >> LocalConfig.kmk
I did not try it as it takes a while to compile... I just linked /usr/lib/virtualbox to /opt/VirtualBox to work around the problem.