This is another hummingbird post, indeed also about the White-necked Jacobin. I thought, when I saw this that it represented one of the flower mite species which live in flowers, compete with hummingbirds for the nectar (apparently they exist in huge numbers), and ride the hummingbirds to get to their next flower.
For higher definition: http://vimeo.com/25437774
Upon close examination, however, this seems to be just a little spider. She's probably still hitching a ride.