Comment 18 for bug 320639

Many of these problems are resolved in 0.99.3-2ubuntu2. However, the pointer is still very hard to control, because the speed varies wildly between its minimum and maximum.

Here is some documentation from the Xorg wiki which confirms my suspicion that both the Xorg core and the synaptics driver are independently applying their own acceleration curves.

http://www.x.org/wiki/Development/Documentation/PointerAcceleration#head-ff75ddde3561d3a8bbb53042a6ea94c4c8958a2c

The solution apparently is to set the synaptics MinSpeed and MaxSpeed values to be _equal_, and let Xorg core do all the acceleration itself.