I was wondering how to create a situation whereby a first-time visitor would see the website overlayed by a "jQuery/Lightbox-esque" modal window warning them that the site contains adult language, and that they could then click "I understand" or something to make it go away.
I don't want to use a separate page to do this (ala, the 'Front Page' module).
It sounds like you would need to store a cookie if you only want "first time visitors" to see this message. Furthermore, it sounds like the front page is not the only candidate for this feature, because visitors may well enter your site from other URLs, from Google or from other sites which link to internal pages on yours.
If the above is correct, then it sounds like you would need custom jQuery code which would run on every page. It would look for the existence (and value) of that cookie and then based on that, decide whether or not to show the message.
And of course when the users clicks that he understands, the jQuery would set the cookie.
That's my first thoughts anyhow.