The concept art and screenshots appear to show a relatively modern game, set perhaps in Renaissance Europe.
The next Thief game, announced as Thief 4, is in development at Eidos Montreal, the studio behind Deus Ex: Human Revolution. A 30-second CG trailer for the game leaked last June. An Eidos Montreal community manager hinted on the Thief 4 forums yesterday that news would be coming today about the game.
You can check out a gallery of 16 images at GamesManiac.
Update: The title is indeed in development for a 2014 launch on PC, PS4 and other next-gen platforms, as was announced in Game Informer's April 2013 cover story.