UPDATE: It's been pointed out by many, many people that 9th April is the release date for the Mythic Map Pack for Halo 3. So it's probably that, as announced by Microsoft this lunchtime. The three-map pack had previously only been available to people who bought Halo Wars, which is why we didn't think of it. Mondays should be banned.
ORIGINAL STORY: Bungie has hinted that something exciting, and presumably Bungie-related, will happen on 9th April, in its weekly website update.
Beyond the reams of interesting chatter about the new Halo 3 Wastelands level, tweaks to prevent "negative EXP boosting" online and a preview of a few technical GDC presentations, the developer gently teases: "P.S. 04/09/09".
Goodness knows what it refers to, but given that Halo 3: ODST is out in the open and the Mythic Map Pack likewise, and that Bungie is known to be working on non-Halo games, it's enough to pique our interest.
Stick it in the diary, we reckon. Or just let us worry about it and enjoy the surprise in two weeks' time. Better be good, Bungie! Probably don't mention it to Don this time, just to be on the safe side.