Chris spotted one of the two remaining unannounced Pyro unlocks in the new Meet The Sniper video. Which is awesome, by the way. For the Demoman’s reaction, the slowly filling jars (also featured on the title card, I notice), and “Yes, yes he did.”
You see the gun fire once, but the muzzle flash is unspecific. There’s a better pic of it over at Chris’s. The shape resembles a flare pistol, but then there are plenty of more exotic devices that you’d probably make like a flare pistol if you had to model them in-game.
Update! clever people were right, it’s a Flare Gun! Full unlockable details and the shocking truth about the big Pyro change blogged over at PC Gamer, plus a few tidbits from Robin Walker on how it’ll all work. I rudely interrupted his game of Defense of the Ancients before breakfast this morning, intending just to say “This sounds ace!” but ended up asking a lot of annoying questions.