Oracle released VirtualBox 4.0.6 today which supports versions 1.9 and 1.10 of xorg-server which is now in portage.
Steps to Reproduce:
1. Upgrade to >= xorg-server-1.9.
2. xorg fails to load vboxvideo module with error "(EE) module ABI major version (8) doesn't match the server's version (10)."
3. Cannot start a KDE session.
Now in CVS. Thanks for reporting.