The incorrect mouse drivers are being used... I have a logitech cordless, but try putting this info into your xorg.conf.
Identifier "Mouse1"
Driver "mouse"
Option "Protocol" "ImPS/2"
Option "ZAxisMapping" "4 5"
Option "Device" "/dev/mouse"
Should do the trick
(The protocol part should be different, and the ZAxisMapping as well). Cheers m8.