I still have the bells ringing in my head, and haven't sit for a moment to put things in order. I'll try to make a common solution to both projects, as the differences are only in the mechanical part of the slider.

Mechanical part:

For the slider, I will use a couple of 1m rails with bearing blocks. On top of the bearing blocks I will attach a plate and a tripod head (later, the pan&tilt mechanism). To move the dolly I'm thinking of a pulley and belt system like those you may find on desktop scanners.

Motorisation:

I have three alternatives: a stepper motor, a regular DC motor (conveniently geared), a modified servo

I was thinking of a stepper controller connected to the parallel port, but if I want to be it an autonomous system I cannot depend on a computer, and laptops usually don't have a parallel port (and to make a microcontroller to drive the parallel port may be well beyond my capacity).

Manual Control:

A DC motor may be easily controlled with a PWM regulator. I may control a servo with a servo tester, but not much contro,l over the speed, etc. To control a stepper motor I will need aditional hardware.

Automated Control:

I am thinking of build a controller with the help of an Arduino board (they are dirty cheap!). I can control DC motors (or servos) with its PWM outputs, shoot the camera when needed and have limit switches. Also I can make it configurable, controlling for how long and how fast I want the motor spinning, number or frequency of shots, etc... and probably attach a LCD panel to make the configuration easier. Still not decided what kind of Arduino board to use, as I think of new features (limiters, triggers, motor control, LCD, etc) probably I'm going to need an Arduino Mega board. To control an stepper motror from the arduino I will need aditional hardware, but I think there are ready-made modules for that.

Pan & Tilt option:

The pan & tilt movement maybe done with a couple of servos. There's no need for full 360º, so no need to modified the servos.

Power supply:

Arduino board don't need much, maybe even a 9V NiMh or a pack of AA batteries will be more than enough. For the motors I have a 12V 4800MAh NiMh battery pack, havent't tested how long it may drive the motors, but probably it will do the job.