How to load a page into a modal dialog?

I want to show my login form in a modal dialog when an anonymous user
tries to post a comment on my site.
Ideally, I'd like to be able to load a page sort of like the $
("somediv)".load function, except I want that div to behave like a
modal div, where it sits on top of everything else, and has a close
button.
Any ideas?
Matt