After update from virtualbox 3.x to 4.x the scroll wheel of my mouse stopped working - just behaved as a three button mouse.
Newer versions of debian no longer have an xorg.conf file - since it's not normally needed.
After some searching I found that it has a config loading directory /usr/share/X11/xorg.conf.d/ and virtualbox had added a 50-vboxmouse.conf there.
I changed this file from
