It now looks like they were being serious, not just dramatic and diva-like. They've got job ads up with the 'writing lead' the most telling.
"Are you a kick-ass writer?" begins their hunt for a writing lead on the official Bungie site. While the other positions all have the same 'wanna take over the world with Bungie?' speech to them, the writing lead post actually revealed something of the project..
"Your words will help define Bungie's next game universe" it continued, finally confirming that Bungie is moving on to a new IP and they're really gearing up for it. Unfortunately that's all the secret spillage the job ads make. Hope it's another shooter, videogamer?
This new fangled creation will have online parts to it, which is no shock given Bungie's appetite for mastering the multiplayer of Halo. They sure do love their stat tracking.