Now with captions! I've also fixed a couple of minor issues involving extra-large images falling out of the viewport and a small gap in the overlay in Firefox. Also, it's more keyboard friendly now. Thanks for all the comments and suggestions.
Grab the latest files here: Lightbox JS
Just a teaser, but per many requests for extended functionality, I am working on a complimentary 'photo-gallery' script. It will include forward/back navigation, preloading, and much more. Check back next week.