In templates/default/layout.html there are elements named "loading" and "close". I don't know if these names are dictated by Lightbox, but unless they are, I'd suggest renaming them with something more closely tied to their use by Simple Lightbox (such as "slb_loading" and "slb_close", as other elements in the template are). The reason is that I just spent a significant amount of time trying to debug a weird problem with my theme, which was actually caused by "loading" being an id used by Simple Lightbox and styled in its CSS. It was obviously easy for me to fix this by changing my element's id, but finding out the cause was time I could have spent with more useful matters.
Thanks for the great plugin, though.