Comment 20 for bug 710762

On oneiric it's enough to run:

gsettings set org.gnome.settings-daemon.peripherals.mouse middle-button-enabled true

that'll make it persistent for the current user, and you can argue about adding it to the "mouse and touchpad" settings GUI by filing a bug against gnome-control-center.

The evdev driver will get a quirk table for known devices (mice, trackballs) that only have two buttons, hopefully we'll have something for oneiric.