http://www.incgamers.com/News/16330/New-Thief-Confirmed
So, uh, yeah. Eidos Montreal's secret project that used the Thief font for their sly mention that they were working on a secret project ages ago that everyone just assumed was Thief 4 but Eidos kept refusing to comment on if it was Thief 4?
It's Thief 4.
Since there's no real info to post, here, have some screenshots of games of Thief past.






And here's an entertaining video of a broken NPC:

http://www.youtube.com/watch?v=3nx3-31fRDw

Edit: Oh, and you can see the actual announcement here:
http://eidosmontreal.com/en/games.html
Currently, they're in pre-production and still recruiting people to work on it, so you can't exactly expect it this holiday season.

Here's the full statement, spell-checked because they're French and don't know English gud:



Eidos-Montréal is excited to unveil, their previously secret second title in development at the studio, as Thief 4, the next installment in the legendary series!

The original Thief title revolutionized 1st person gaming and defined a new genre of stealth based game play. Thief has consistently met with critical acclaim, with the revered series winning awards for its originality in gameplay and sound design. Thief’s list of accolades includes a birth in GameSpy’s Hall of Fame and GameSpot citing Thief as one of the Greatest Games of All Time. Thief’s script and ground breaking sound immersion design was recognized by The Academy of Interactive Arts and Sciences and was awarded its prestigious Outstanding Achievement in Character and Story Development award and nominated for Outstanding Achievement in Sound Design.

"We're in the early development stages for Thief 4, but this is an incredibly ambitious project and a very exciting one. It's too early for us to offer any specific game details, right now we are focused on recruiting the very best talent to join the core team at the studio and help us make, what we believe will be one of the most exciting games on the market."