From VirtualBox-4.2.2 Changelog (https://www.virtualbox.org/wiki/Changelog) [...] Linux Additions: support X.Org Server 1.13 Removing "<x11-base/xorg-server-1.12.99[-minimal] dependency from x11-drivers/xf86-video-virtual-box-4.2.2 and installing vboxvideo_drv_113.so seems to work for me.
Weird, have modded that line to just xorg-server -minimal, and upgraded to 1.13 - rebuild _all_ virtualbox modules, vboxvideo driver (and evdev), and X will not start, with an ABI mismatch of vboxvideo in the log as the culprit.
Sorry for the long delay guys... As soon as my dev machine is back online (should be in November 9th) I will fix this bug.
+*xf86-video-virtualbox-4.2.4 (10 Nov 2012) + + 10 Nov 2012; Lars Wendler <polynomial-c@gentoo.org> + -xf86-video-virtualbox-4.2.0.ebuild, xf86-video-virtualbox-4.2.2.ebuild, + +xf86-video-virtualbox-4.2.4.ebuild: + Version bump. Removed old. Moved restriction for xorg up to 1.13.x (bug + #439482). +