When started normally it fails with a pop saying "Callee RC: NS_ERROR_FACTORY_NOT_REGISTERED (0x80040154)" This bug seems to do with a hardened build and location of binaries. Fact is it does not work when starting /usr/bin/VirtualBox (the symlink) but does work fine when starting /opt/VirtualBox/VirtualBox Also see http://www.virtualbox.org/ticket/3672#comment:28
Please post your `emerge --info' in a comment.
Created attachment 257158 [details] emerge --info
I can reproduce this same error if I immediately try to launch "VirtualBox" without relogging (or doing "source /etc/profile") for the new VBOX_APP_HOME="/opt/VirtualBox" variable from /etc/env.d/90virtualbox to take effect. Does this error still happen if that variable is set in the environment from which you call VirtualBox?
If you are still able to reproduce this after relogging (so proper env vars are set), please reopen.