Sorry for sticking with RC-Servo. No experience with steppers (but some with arduino).
An endless RC servo is speed controllable by using a very cheap "servo tester" -> http://bit.ly/10U6a7T If you want to get fancy, you can use you sound card output from your Laptop to generate a signal that the servo understands (Pulse Code Modulation) and interconnect the servo to signal and battery. I have actually never done it that way, but it should work. by that you could program the speed for re-doable slides.
Mounting a big gear on top of the servo and using rubber belts gives smooth (though not very speedy) movements. The torque of those servos is amazing.
This was just a quick and dirty construction but should give an impression of the possibilities. I am just a hobbyist, so looking forward for feedback.
RC Slider Dolly sample shot:
Pics Slider: https://www.dropbox.com/s/a76bmauzkpbwcg7/2013-03-17%2022.35.56.jpg
Pics Servo Puller: https://www.dropbox.com/s/rlptfoz07ew1wd2/2013-03-17%2022.36.04.jpg
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