I'm pretty sure you already saw this article, but I will post it anyways. Using this technique, it will be virtually impossible for a robot to register or perform any other spam related tasks. Why? Because you can actually set yourself the increment value, for example. On top of that, you can generate easy a slider with jQuery. I would really want this present into Xen. It will bring value to the product, since is something not implemented into any software out there.