It all started with a mysterious ratings classification in Australia for Uncharted: Fight for Fortune (via All Games Beta). The listing confirms that One Loop Games is working on a Sony-published Uncharted game that features very mild amounts of violence and drug use.
Intrepid NeoGAF users took this news and scoured the Internet for more info, eventually coming across yet another ratings classification, this time in Brazil. The translated page reveals that Fight for Fortune is bound for the PlayStation Vita and that it's a casino/card game.
Sony has yet to make any official announcements, but I'd say that a tweet from company executive Shuhei Yoshida about Fight for Fortune is pretty concrete confirmation of its existence. One Loop's website mentions two upcoming projects, one of which is already complete. Could that be the Vita-bound Uncharted game? We'll have to wait and see.