Hi @All 
thanks Mytx for your info about your project.
(he answered to my project info/ request)
i have an motorfader - and a arduino sketch that controls it.
My actual working state is as following:
you can set the fader to a position:
driveTo(900); // 0..1023
the software runs a speed/ distance map.
so if the fader must move a long way it will drive with maximum speed - and when it comes nearer to the target position it will slow down.
with this system i get a relative small tolerances. ( i think its 2% - on an 100mm fader means 2mm 
)
at the moment i work on the packaging into an arduino lib.
will be ready the next days. (i hope )
I used this fader:
Sparkfun Slide Pot - Motorized (10k Linear Taper)
and there are also some with audio taper profiles:
Sparkfun Slide Pot - Motorized (10k Audio Taper)
this things are from Top-Up-Industries
an other motorized fader from ALPS
as driver i used the Arduino Motor Shield
with this i can drive two faders.
my next step on the hardware side will be also to use an L293D QUADRUPLE HALF-H DRIVER from TI
it has the Output Clamp Diodes build in - thats really cool - so you dont need so much things around. (i just missing the inverter thing)
sunny greetings
stefan