A couple of years ago my roommate and I put together a home-build MoCo linear stage dolly for doing time lapses (see photos and videos below). We did some research and came up with the simplest design we could. We used a 7 foot section of 1x6 plank with PVC rails and PVC T-sections cut in half as sliders along the rails. We put a bit of felt underneath the T-sections to make it slide a bit smoother. Underneath the chassis was a dc-motor with spool that would turn and wind itself in either direction.
For the control side of things, we used an Arduino Duemilanove to control the motor direction/speed. I made a simple python UI to control things from a PC.
I have plans to revive this project into something more robust and with more functionality. But, that will probably have to wait until after I graduate.