It's custom made from an engineering company. Is is basically an aluminum track with a large steel threaded rod through the center. This is then power by a powerful stepping motor which is controlled by a pc. We then mounted a 150 mm tripod cup and attached the tripod head directly to it.

The cool thing about it is that it can be set at any angle including vertically and it can travel as fast or as slow as you like. You can also mount the camera underneath to get super low shots. The BBC used something similar for a lot of the Planet Earth stuff.

Here's another picture with the setup (it's Pawel Achtel in the picture)