Why not lightbox? Suggestions?

Why not lightbox? Suggestions?

Is there a reason why jQuery UI still lacks one of the most fundamental new UI elements in recent web design: the lightbox?

I know there's dialog but AFAIK that's not quite the same thing. For example, I've been asked to force users to click 'Agree' to a lightbox prompt when moving away from a page (through clicking a link on the page, for example). I need to put the T&Cs in the lightbox and there doesn't seem to be an option for that with the dialog widget.

What would be people say is the best lightbox solution for jQuery? Bear in mind that I'm already loading the jQuery core library, the jQuery UI library with accordion support. So I don't want to add too many more HTTP requests or much more file size.

Thanks
pd