Comment 38 for bug 426675

It still doesn't work on Precise.
The "left-handed" option in the system settings has no effect.
I even changed the value of "left-handed" to "true" in the following file.
～/.gconf/desktop/gnome/peripherals/mouse/%gconf.xml
However, it didn't change anything.

BTW, I was using Arch with gnome 3 before. The left-handed option has been of no use since that time.
I installed ubuntu 12.04 yesterday in place of arch and attached the previous "/home" partition to the new system.
The first time I logged into the desktop, the left-handed option was selected by default because of the old config file, but the actual behavior of the mouse was still right-handed.

Nevertheless, I "tried ubuntu without installing" in the U-disk, the option worked.