I've noticed in a recent project that Safari 3.1.2 likes to hold an iFrame in its cache by id (in a lightframe's case "Lightboxframe") - I had to add a random number variable at the top of lightbox.js to avoid getting the same content in each frame. Makes sense to have these individual anyway as they are ids.

var idrnd = Math.floor(Math.random()*101);

and append this to each instance of the id (there are 8)

<iframe id="lightboxframe'+idrnd+'"
and
$('#lightboxframe'+idrnd+'')

Obviously it may be best for Drupal to pass the nid as a variable to the js and append that, I may submit a patch, but feel free to pick up.