I never realized how inconvenient (and annoying) it is to use a mouse without a scroll wheel.
It took me a bit of searching google, forums, mailing lists, howto's, and what not, to figure out how to get the scroll wheel on my Microsoft (PS/2) Optical Intellimouse to work under xorg/x11, on the Slackware Linux OS.
Most (newer) distros already come with these settings by default, but Slackware does not.
I'd rather not use another distro just to get the scroll wheel on the mouse to work, if you know what I mean.
Since I don't want to forget these settings, or repeat that hanus search effort, I will place the configuration (snippet) here for (my own) future reference.
# file: /etc/X11/xorg.conf
Option "Protocol" "Auto"
Option "ZAxisMapping" "4 5"
Option "Device" "/dev/psaux"
The configuration above is simply a snippet of the Mouse Configuration within the /etc/X11/xorg.conf file, and is not the entire configuration.
You may want to delete your current mouse settings entirely, and replace them with the snippet above, or you can simply use the snippet above as reference on how to modify your configuration to get the scroll wheel working correctly.