After plugging in a Razer Diamondback 3g in Ubuntu 11.10 I found that the sensitivity was spectacularly high, and I was unable to edit it using gnome settings.
The fix is to manually edit xorg.conf to set a constant deceleration for the device in question.
sudo gedit /etc/X11/xorg.confWhether or not the file exists, simply append the following:
Identifier "mouse speed adjustment"
MatchProduct "Diamondback 3G"
Option "ConstantDeceleration" "2"
Where MatchProduct will look for a device name containing what is specified. This can be found by running:
cat /proc/bus/input/devicesand scanning the resulting output for the name of the device in question.
The higher the number associated with ConstantDeceleration, the less sensitive the mouse will be.
Hope this helps.