For those interested in building 6DoF motion platforms, you may want to check out my new linear actuator design.
It makes use of timing belts and pulleys, and has bungee cord for balancing the payload.

I have made a video that shows the basic structure and initial tests. Linear Actuator Design - YouTube

For further details, you can check my website: DIY Motion Platform V
The google sketchup file that you can download can be used to check all dimensions.