We've got news that Eidos has filed for trademark of the phrase "Tomb Raider Underworld" with the United States Patent and Trademark Office.
The word "Underworld" might hint at possible Lara Croft criminal ties in the upcoming sequel. But it might also refer to some dungeons or underground locations.
Considering the way we found out about this, it is no surprise that we don't have further details right now. We don't expect more news for quite a while (at least not until Eidos releases an official announcement), but we'll keep you informed with any updates we find out.