We've put requests into Bungie and Microsoft. Read Bungie pres Harold Ryan's message after the break. (Note there's no mention of the publisher; looks like they want to keep it clean).
[Update: We just heard back from Bungie's Luke "Luke Smith" Smith who tells us, "We still have an exclusive publishing agreement with Microsoft, as we confirmed when we announced the split last year. That arrangement has not changed." So, there you have it. No word back from the Big M. Update 2: Luke's keeping busy. He left two comments on NeoGAF (here and here) not only assuring skeptics that this was in fact not a part of the game's marketing plan ("not our style") and that Bungie's publisher is, indeed, Microsoft.
Update 3: Those sleuths at X3F have noticed a sort of reversal in Bungie's teasing: references to the superintendent in Halo 3 have been removed. Update 4: We interviewed Microsoft Game Studio's Phil Spencer earlier this afternoon, just before this news broke, but conveniently we had already asked him about the absence of Bungie's game at the Microsoft press conference. Read his comments here.
Update 5: Xboss Don Mattrick has confirmed that Bungie is in fact working on a new Halo title. Whether or not this unannounced project is that title is unclear. Update 6 (17 July): Mattrick tells the L.A. Times the Bungie announcements was cut to -- get this -- save time. After all, they had an "embarrassment of riches" as it was.]